[Reland] [libc++] Don't alias quick_exit if __ANDROID_API__ < 21

[thomasanderson]

Summary

quick_exit() and at_quick_exit() were introduced in android NDK 21:
https://android.googlesource.com/platform/prebuilts/ndk/+/dev/platform/sysroot/usr/include/stdlib.h#55

This CL conditions _LIBCPP_HAS_QUICK_EXIT on __ANDROID_API__ >= 21. The only place this macro is used is in some using declarations: using ::quick_exit, using ::at_quick_exit.

Also, add a missing include to sys/cdefs.h which is what defines __BIONIC__.

This is a reland of r317124, which caused a circular dependency on Darwin due to the <sys/cdefs.h> include. The solution is to only include this header on Linux.

[thomasanderson]

danalbert@ ptal.

It seems I ran into this bug https://bugs.llvm.org/show_bug.cgi?id=25341
But, I was unable to reproduce the issue when building on a Mac machine, so I'm not 100% sure it's fixed. Is there a way to run the Darwin bot on this CL? Also, I'm not sure if the __libcpp_has_include is necessary any more.

[thomasanderson]

Friendly ping

[EricWF]

LGTM.

include/__config

 // Need to detect which libc we're using if we're on Linux.
 #if defined(__linux__)
 #include <features.h>
+#if __libcpp_has_include(<sys/cdefs.h>)
+// Needed to detect bionic libc.
+#include <sys/cdefs.h>
+#endif
 #if !defined(__GLIBC_PREREQ)
 #define __GLIBC_PREREQ(a, b) 0
 #endif // !defined(__GLIBC_PREREQ)

 #define _LIBCPP_HAS_C11_FEATURES
 #elif defined(__linux__)
 #if !defined(_LIBCPP_HAS_MUSL_LIBC)
-#if __GLIBC_PREREQ(2, 15) || defined(__BIONIC__)
+#if __GLIBC_PREREQ(2, 15) || (defined(__BIONIC__) && (__ANDROID_API__ >= 21))
 #define _LIBCPP_HAS_QUICK_EXIT
 #endif
 #if __GLIBC_PREREQ(2, 17)