Abstract rwlocks into a class, provide a SRW lock implementation for windows

This requires _WIN32_WINNT >= 0x0600.

If someone wants to spend effort on supporting earlier versions,
one can easily add another fallback implementation based on
critical sections, or try to load SRW lock functions dynamically.

This makes sure that the FDE cache is thread safe on windows.

Differential Revision: https://reviews.llvm.org/D38704

llvm-svn: 316364