This has been obsoleted by C++ thread_local for a long time.
As far as I know, Xcode was the last supported toolchain to add
support for C++ thread_local in 2016.
As a precaution, use LLVM_THREAD_LOCAL which provides even greater
backwards compatibility, allowing this to function even pre-C++11
versions of GCC.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D141349
e4e0f93307