Insert a SmartMutex templated class into the class hierarchy, which takes a template...

Insert a SmartMutex templated class into the class hierarchy, which takes a template parameter specifying whether this mutex
should become a no-op when not running in multithreaded mode.  Make sys::Mutex a typedef of SmartMutex<false>, to preserve source compatibility.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73709 91177308-0d34-0410-b5e6-96231b3b80d8