Use std::mutex to avoid memory allocation after OOM

ManagedStatic<sys::Mutex> would lazilly allocate a sys::Mutex to lock
when reporting an OOM, which is a bad idea.

The three STL implementations that I know of use pthread_mutex_lock and
EnterCriticalSection to implement std::mutex. I'm pretty sure that
neither of those allocate heap memory.

It seems that we unconditionally use std::mutex without testing
LLVM_ENABLE_THREADS elsewhere in the codebase, so this should be
portable.

llvm-svn: 307827