MS inline asm: Use memory constraints for functions instead of registers

This is consistent with how we parse them in a standalone .s file, and
inline assembly shouldn't differ.

This fixes errors about requiring more registers than available in
cases like this:
  void f();
  void __declspec(naked) g() {
    __asm pusha
    __asm call f
    __asm popa
    __asm ret
  }

There are no registers available to pass the address of 'f' into the asm
blob.  The asm should now directly call 'f'.

Tests will land in Clang shortly.

llvm-svn: 214550