[MSVC Compat] Permit conversions from pointer-to-function to pointer-to-object...

[MSVC Compat] Permit conversions from pointer-to-function to pointer-to-object iff -fms-compatibility

We permit implicit conversion from pointer-to-function to
pointer-to-object when -fms-extensions is specified.  This is rather
unfortunate, move this into -fms-compatibility and only permit it within
system headers unless -Wno-error=microsoft-cast is specified.

llvm-svn: 251738