Remove the ThreadLocal template from LLVM.

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