[clangd] Use pthread instead of thread_local to support more runtimes.

Summary:
thread_local has nice syntax and semantics, but requires __cxa_thread_atexit,
and some not-ancient runtime libraries don't provide it.
The clang-x86_64-linux-selfhost-modules buildbot is one example :-)

It works on windows, and the other platforms clang-tools-extra supports should
all have the relevant pthread API. So we just use that if it's available,
falling back to thread_local (so if a platform has neither, we'll fail to link).

The fallback should really be the other way, that would require cmake changes.

Reviewers: ilya-biryukov, bkramer

Subscribers: klimek, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D42742

llvm-svn: 323949