[libc++abi] Add temporary workaround to unblock Chrome

Chrome rolls libc++ and libc++abi as separate projects. As a result, they
may not always be updated in lockstep, and this can lead to build failures
when mixing libc++ that doesn't have <__thread/support.h> with libc++abi
that requires it.

This patch adds a workaround to make libc++abi work with both versions.
While Chrome's setup is not supported, this workaround will allow them
to go back to green and do the required work needed to roll libc++ and
libc++abi in lockstep. This workaround will be short-lived -- I have a
reminder to go back and remove it by EOW.

libcxxabi/src/cxa_exception_storage.cpp

@@ -12,7 +12,12 @@
 
 #include "cxa_exception.h"
 
-#include <__thread/support.h>
+// TODO: Temporary workaround, see https://github.com/llvm/llvm-project/pull/79654#issuecomment-1919397302
+#if __has_include(<__thread/support.h>)
+#  include <__thread/support.h>
+#else
+#  include <__threading_support>
+#endif
 
 #if defined(_LIBCXXABI_HAS_NO_THREADS)
 

libcxxabi/src/cxa_guard_impl.h

@@ -55,7 +55,13 @@
 #  endif
 #endif
 
-#include <__thread/support.h>
+// TODO: Temporary workaround, see https://github.com/llvm/llvm-project/pull/79654#issuecomment-1919397302
+#if __has_include(<__thread/support.h>)
+#  include <__thread/support.h>
+#else
+#  include <__threading_support>
+#endif
+
 #include <cstdint>
 #include <cstring>
 #include <limits.h>

libcxxabi/src/cxa_thread_atexit.cpp

@@ -8,7 +8,12 @@
 
 #include "abort_message.h"
 #include "cxxabi.h"
-#include <__thread/support.h>
+// TODO: Temporary workaround, see https://github.com/llvm/llvm-project/pull/79654#issuecomment-1919397302
+#if __has_include(<__thread/support.h>)
+#  include <__thread/support.h>
+#else
+#  include <__threading_support>
+#endif
 #ifndef _LIBCXXABI_HAS_NO_THREADS
 #if defined(__ELF__) && defined(_LIBCXXABI_LINK_PTHREAD_LIB)
 #pragma comment(lib, "pthread")

libcxxabi/src/fallback_malloc.cpp

@@ -9,7 +9,12 @@
 #include "fallback_malloc.h"
 #include "abort_message.h"
 
-#include <__thread/support.h>
+// TODO: Temporary workaround, see https://github.com/llvm/llvm-project/pull/79654#issuecomment-1919397302
+#if __has_include(<__thread/support.h>)
+#  include <__thread/support.h>
+#else
+#  include <__threading_support>
+#endif
 #ifndef _LIBCXXABI_HAS_NO_THREADS
 #if defined(__ELF__) && defined(_LIBCXXABI_LINK_PTHREAD_LIB)
 #pragma comment(lib, "pthread")

libcxxabi/test/test_fallback_malloc.pass.cpp

@@ -11,7 +11,12 @@
 #include <cassert>
 #include <inttypes.h>
 
-#include <__thread/support.h>
+// TODO: Temporary workaround, see https://github.com/llvm/llvm-project/pull/79654#issuecomment-1919397302
+#if __has_include(<__thread/support.h>)
+#  include <__thread/support.h>
+#else
+#  include <__threading_support>
+#endif
 
 // UNSUPPORTED: c++03
 // UNSUPPORTED: modules-build && no-threads