Fix Windows build after llgs upstream.

With _HAS_EXCEPTIONS=0, Windows' version of <thread> will fail to
compile because it calls __uncaught_exception(), which is compiled
out due to _HAS_EXCEPTIONS=0.  This just creates a stub version
of __uncaught_exception() which always fails.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@212076 91177308-0d34-0410-b5e6-96231b3b80d8