[Support] Wrap extern TLS variable in getter function
This patch wraps an external thread local storage variable inside of a
getter function and makes it have internal linkage. This allows LLVM to
be built with BUILD_SHARED_LIBS on windows with MinGW. Additionally it
allows Clang versions prior to 10 to compile current trunk for MinGW.
Differential Revision: https://reviews.llvm.org/D73639