[MSVC Compatibility] Permit static_cast from void-ptr to function-ptr

The MSVC 2013 and 2015 implementation of std::atomic is specialized for
pointer types.  The member functions are implemented using a static_cast
from void-ptr to function-ptr which is not allowed in the standard.
Permit this conversion if -fms-compatibility is present.

This fixes PR23733.

llvm-svn: 238877