[PECOFF] Use RAII object for mutex.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@204853 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Avoid C-style cast.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@204855 91177308-0d34-0410-b5e6-96231b3b80d8