Make __has_extension(c_thread_local) work.

Unlike C++11's "thread_local" keyword, C11's "_Thread_local" is in the
reserved namespace, meaning we provide it unconditionally; it is marked
as KEYALL in TokenKinds.def.

This means that like all the other C11 keywords, we can expose its
presence through __has_extension().

llvm-svn: 190755