Support/Unix: use ScopedLock wherever possible

Only one function remains a bit too complicated
for a simple mutex guard. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216335 91177308-0d34-0410-b5e6-96231b3b80d8
Support: make LLVM Mutexes STL-compatible

Use lock/unlock() convention instead of acquire/release().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216336 91177308-0d34-0410-b5e6-96231b3b80d8