[libc++] Clean up old macOS back-deployment workarounds

[ldionne]

Summary

This patch bumps the minimum macOS version for building the dylib
or back-deploying a statically-linked libc++ from macOS 10.11 to
macOS 10.13. AFAICT, Chrome was the last one to require macOS 10.11,
but since then they have bumped their minimal supported version to
macOS 10.13.

[philnik]

Thanks for the cleanup!

libcxx/docs/ReleaseNotes.rst

95

[ldionne]

Announced here on Discord: https://discourse.llvm.org/t/libc-bumping-minimal-deployment-target-for-building-the-dylib-static-library-on-macos/68912

Will ship on March 10th unless there's new information.

libcxx/docs/ReleaseNotes.rst

 API Changes
 -----------
 
 ABI Affecting Changes
 ---------------------
 
 Build System Changes
 --------------------
+
+- Building libc++ and libc++abi for Apple platforms now requires targeting macOS 10.13 and later.
+  This is relevant for vendors building the libc++ shared library and for folks statically linking
+  libc++ into an application that has back-deployment requirements on Apple platforms.

libcxx/docs/index.rst

 GCC          12              In C++11 or later only     latest stable release per `GCC's release page <https://gcc.gnu.org/releases.html>`_
 ============ =============== ========================== =====================
 
 Libc++ also supports common platforms and architectures:
 
 =============== ========================= ============================
 Target platform Target architecture       Notes
 =============== ========================= ============================
-macOS 10.9+     i386, x86_64, arm64       Building the shared library itself requires targetting macOS 10.11+
+macOS 10.9+     i386, x86_64, arm64       Building the shared library itself requires targetting macOS 10.13+
 FreeBSD 12+     i386, x86_64, arm
 Linux           i386, x86_64, arm, arm64  Only glibc-2.24 and later and no other libc is officially supported
 Windows         i386, x86_64              Both MSVC and MinGW style environments
 AIX 7.2TL5+     powerpc, powerpc64
 =============== ========================= ============================
 
 Generally speaking, libc++ should work on any platform that provides a fairly complete
 C Standard Library. It is also possible to turn off parts of the library for use on

libcxx/src/chrono.cpp

 //  better for it to not exist and have the rest of libc++ use system_clock
 //  instead.
 //
 
 #ifndef _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 
 #if defined(__APPLE__)
 
-// TODO(ldionne):
-// This old implementation of steady_clock is retained until Chrome drops supports
-// for macOS < 10.12. The issue is that they link libc++ statically into their
-// application, which means that libc++ must support being built for such deployment
-// targets. See https://llvm.org/D74489 for details.
-#if (defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 101200) || \
-    (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ < 100000) || \
-    (defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ < 100000) || \
-    (defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ < 30000)
-# define _LIBCPP_USE_OLD_MACH_ABSOLUTE_TIME
-#endif
-
-#if defined(_LIBCPP_USE_OLD_MACH_ABSOLUTE_TIME)
-
-//   mach_absolute_time() * MachInfo.numer / MachInfo.denom is the number of
-//   nanoseconds since the computer booted up.  MachInfo.numer and MachInfo.denom
-//   are run time constants supplied by the OS.  This clock has no relationship
-//   to the Gregorian calendar.  It's main use is as a high resolution timer.
-
-// MachInfo.numer / MachInfo.denom is often 1 on the latest equipment.  Specialize
-//   for that case as an optimization.
-
-static steady_clock::rep steady_simplified() {
-    return static_cast<steady_clock::rep>(mach_absolute_time());
-}
-static double compute_steady_factor() {
-    mach_timebase_info_data_t MachInfo;
-    mach_timebase_info(&MachInfo);
-    return static_cast<double>(MachInfo.numer) / MachInfo.denom;
-}
-
-static steady_clock::rep steady_full() {
-    static const double factor = compute_steady_factor();
-    return static_cast<steady_clock::rep>(mach_absolute_time() * factor);
-}
-
-typedef steady_clock::rep (*FP)();
-
-static FP init_steady_clock() {
-    mach_timebase_info_data_t MachInfo;
-    mach_timebase_info(&MachInfo);
-    if (MachInfo.numer == MachInfo.denom)
-        return &steady_simplified;
-    return &steady_full;
-}
-
-static steady_clock::time_point __libcpp_steady_clock_now() {
-    static FP fp = init_steady_clock();
-    return steady_clock::time_point(steady_clock::duration(fp()));
-}
-
-#else // vvvvv default behavior for Apple platforms  vvvvv
-
 // On Apple platforms, only CLOCK_UPTIME_RAW, CLOCK_MONOTONIC_RAW or
 // mach_absolute_time are able to time functions in the nanosecond range.
 // Furthermore, only CLOCK_MONOTONIC_RAW is truly monotonic, because it
 // also counts cycles when the system is asleep. Thus, it is the only
 // acceptable implementation of steady_clock.
 static steady_clock::time_point __libcpp_steady_clock_now() {
     struct timespec tp;
     if (0 != clock_gettime(CLOCK_MONOTONIC_RAW, &tp))
         __throw_system_error(errno, "clock_gettime(CLOCK_MONOTONIC_RAW) failed");
     return steady_clock::time_point(seconds(tp.tv_sec) + nanoseconds(tp.tv_nsec));
 }
 
-#endif
-
 #elif defined(_LIBCPP_WIN32API)
 
 // https://msdn.microsoft.com/en-us/library/windows/desktop/ms644905(v=vs.85).aspx says:
 //    If the function fails, the return value is zero. <snip>
 //    On systems that run Windows XP or later, the function will always succeed
 //      and will thus never return zero.
 
 static LARGE_INTEGER

libcxx/src/filesystem/filesystem_common.h

 # include <dirent.h>   // for DIR & friends
 # include <fcntl.h>    /* values for fchmodat */
 # include <sys/stat.h>
 # include <sys/statvfs.h>
 # include <sys/time.h> // for ::utimes as used in __last_write_time
 # include <unistd.h>
 #endif // defined(_LIBCPP_WIN32API)
 
-#include "../include/apple_availability.h"
-
-#if !defined(__APPLE__)
-// We can use the presence of UTIME_OMIT to detect platforms that provide
-// utimensat.
+// We can use the presence of UTIME_OMIT to detect platforms that provide utimensat.
 #if defined(UTIME_OMIT)
-#define _LIBCPP_USE_UTIMENSAT
+# define _LIBCPP_USE_UTIMENSAT
 #endif
-#endif
 
 // TODO: Check whether these functions actually need internal linkage, or if they can be made normal header functions
 _LIBCPP_DIAGNOSTIC_PUSH
 _LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Wunused-function")
 _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wunused-function")
 _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wunused-template")
 
 #if defined(_LIBCPP_WIN32API)

libcxx/src/include/apple_availability.h

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP_SRC_INCLUDE_APPLE_AVAILABILITY_H
 #define _LIBCPP_SRC_INCLUDE_APPLE_AVAILABILITY_H
 
 #if defined(__APPLE__)
 
 #if defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
-#if __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101300
-#define _LIBCPP_USE_UTIMENSAT
-#endif
-#elif defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__)
-#if __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 110000
-#define _LIBCPP_USE_UTIMENSAT
-#endif
-#elif defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__)
-#if __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ >= 110000
-#define _LIBCPP_USE_UTIMENSAT
-#endif
-#elif defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__)
-#if __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ >= 40000
-#define _LIBCPP_USE_UTIMENSAT
-#endif
-#endif // __ENVIRONMENT_.*_VERSION_MIN_REQUIRED__
-
-#if defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
 #if __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101500
 #define _LIBCPP_USE_ULOCK
 #endif
 #elif defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__)
 #if __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 130000
 #define _LIBCPP_USE_ULOCK
 #endif
 #elif defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__)