[PECOFF] Use std::mutex.

Use <mutex> instead of "llvm/Support/Mutex.h".

Also change the type of mutex for the context object to recursive mutex, as
the driver could acquire the lock recursively. E.g. If file A has .drectve
section containing /defaultlib:B, the driver tries to parse file B, and if
file B has .drectve section, the driver acquires the lock again.

llvm-svn: 204850