[Support] Access threadIndex via a wrapper function

On Unix platforms, this wrapper function is inline, so it should
expand to the same direct access to the thread local variable. On
Windows, it's a non-inline function within Parallel.cpp, allowing
making the thread_local variable static.

Windows Native TLS doesn't support direct access to thread local
variables in a different DLL, and GCC/binutils on Windows occasionally
has problems with non-static thread local variables too.

This fixes mingw dylib builds with native TLS after
e6aebff6.

At the same time, move the whole thread local variable within
    #if LLVM_ENABLE_THREADS
to fix builds without threading support.

Differential Revision: https://reviews.llvm.org/D133759