[libc++][format] Removes format sources.

[Mordante]

Summary

The source file is used to anchor the destructor of format_error. When
format is moved from experimental to stable this code would move to the
dylib. One issue with code in the dylib is that it can't be used in
constexpr context. There is a proposal to make format work during
constant evaluation

P2758 Emitting messages at compile time

This paper has initially been received favourable by EWG. Therefore move
the code to the header. This also avoids possible availability issues on
Mac back deployment targets.

Note it is expected that format will no longer be experimental with the
next LLVM release.

[Mordante]

Rebased and CI fixes.

[Mordante]

Fixes CI.

[ldionne]

Nice cleanup, thanks!

libcxx/CMakeLists.txt

target_compile_definitions(${target} PRIVATE
                                  _CRTBLD
                                  # Don't warn on the use of "deprecated"
                                  # "insecure" functions which are standards
                                  # specified.
                                  _CRT_SECURE_NO_WARNINGS
                                  # Use the ISO conforming behaviour for conversion
                                  # in printf, scanf.
                                  _CRT_STDIO_ISO_WIDE_SPECIFIERS)
-    # Clang-cl shared builds don't support the experimental library.
-    # To avoid linker errors the format_error destructor is inlined for the
-    # dylib. Users can never use format in this mode.
-    # TODO FMT Remove when format becomes mainline.
-    if (LIBCXX_ENABLE_SHARED)
-      target_compile_definitions(${target} PRIVATE
-                                 _LIBCPP_INLINE_FORMAT_ERROR_DTOR)
-    endif()
   endif()
 endfunction()
 
 # Configuration file flags =====================================================
 config_define(${LIBCXX_ABI_VERSION} _LIBCPP_ABI_VERSION)
 config_define(${LIBCXX_ABI_NAMESPACE} _LIBCPP_ABI_NAMESPACE)
 config_define_if(LIBCXX_ABI_FORCE_ITANIUM _LIBCPP_ABI_FORCE_ITANIUM)
 config_define_if(LIBCXX_ABI_FORCE_MICROSOFT _LIBCPP_ABI_FORCE_MICROSOFT)

libcxx/include/__format/format_error.h

 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20
 
+_LIBCPP_DIAGNOSTIC_PUSH
+_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wweak-vtables")
 class _LIBCPP_EXCEPTION_ABI format_error : public runtime_error {
 public:
   _LIBCPP_HIDE_FROM_ABI explicit format_error(const string& __s)
       : runtime_error(__s) {}
   _LIBCPP_HIDE_FROM_ABI explicit format_error(const char* __s)
       : runtime_error(__s) {}
-  // TODO FMT Remove when format is no longer experimental.
-  // Avoids linker errors when building the Clang-cl Windows DLL which doesn't
-  // support the experimental library.
-#  ifndef _LIBCPP_INLINE_FORMAT_ERROR_DTOR
-  ~format_error() noexcept override;
-#  else
-  ~format_error() noexcept  override {}
-#  endif
+  _LIBCPP_HIDE_FROM_ABI_VIRTUAL
+  ~format_error() noexcept override = default;
 };
+_LIBCPP_DIAGNOSTIC_POP
 
 _LIBCPP_NORETURN inline _LIBCPP_HIDE_FROM_ABI void
 __throw_format_error(const char* __s) {
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
   throw format_error(__s);
 #else
   (void)__s;
   _VSTD::abort();
 #endif
 }
 
 #endif //_LIBCPP_STD_VER >= 20
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FORMAT_FORMAT_ERROR_H

libcxx/src/CMakeLists.txt

if (LIBCXX_ENABLE_STATIC)
   endif()
 endif()
 
 # Add a meta-target for both libraries.
 add_custom_target(cxx DEPENDS ${LIBCXX_BUILD_TARGETS})
 
 set(LIBCXX_EXPERIMENTAL_SOURCES
   experimental/memory_resource.cpp
-  format.cpp
   )
 
 add_library(cxx_experimental STATIC ${LIBCXX_EXPERIMENTAL_SOURCES})
 target_link_libraries(cxx_experimental PUBLIC cxx-headers)
 if (LIBCXX_ENABLE_SHARED)
   target_link_libraries(cxx_experimental PRIVATE cxx_shared)
 else()
   target_link_libraries(cxx_experimental PRIVATE cxx_static)

libcxx/src/format.cpp

-//===----------------------------------------------------------------------===//
-//
-// 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
-//
-//===----------------------------------------------------------------------===//
-
-#include <format>
-
-_LIBCPP_BEGIN_NAMESPACE_STD
-
-#ifndef _LIBCPP_INLINE_FORMAT_ERROR_DTOR
-format_error::~format_error() noexcept = default;
-#endif
-
-_LIBCPP_END_NAMESPACE_STD