Change libclang initialization to use std::call_once instead of
hand rolled once-initialization, and rename the mutex to be more
descriptive of its actual purpose.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@211121 91177308-0d34-0410-b5e6-96231b3b80d8
Remove dead code.

The parsing for -Rpass= had been factored into the function
GenerateOptimizationRemarkRegex, but at the time I forgot to remove
the original code that just handled OPT_Rpass_EQ.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@211122 91177308-0d34-0410-b5e6-96231b3b80d8