Don't bring in llvm/Support/thread.h in Threading.cpp

Doing so defines the type llvm::thread.  On FreeBSD, we need
to call a macro which references its own ::thread type, which
causes an ambiguity due to ADL when inside of the llvm namespace.

Since we don't even need this unless LLVM_ENABLE_THREADS == 1,
we don't even need this type anyway, as it is always equal to
std::thread, so we can just use that directly.

llvm-svn: 296891