[Support/LockFileManager] Re-apply r203137 and r203138 but use symbolic links only on unix.

Reid Kleckner pointed out that we can't use symbolic links on Windows.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203162 91177308-0d34-0410-b5e6-96231b3b80d8