Add a Mutex class for thread synchronization in a platform-independent way.
The current implementation only supports pthreads. Win32 use of Critical
Sections will be added later. The design permits other threading models to
be used if (and only if) pthreads is not available.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22403 91177308-0d34-0410-b5e6-96231b3b80d8
