The implementation of the Lock class on Windows currently uses C++
mutexes. That introduces a dependency on the C++ runtime on that
platform.
Use a Windows CriticalSection instead of a std::mutex to avoid that
dependency.
This works for me with MinGW (tested in a CLANG64 environment of MSYS2).
See also D126291.
Differential Revision: https://reviews.llvm.org/D127316