Fix failing test valist.c for windows

[john-h-kastner]

Even though references don't exist in C, va_list is a typedef of
__builtin_va_list & on windows. In order to generate correct constraints
for var arg functions on windows, we need to strip the reference type.

[mattmccutchen-cci]

Thanks! Where does the reference type come from? Is Clang generating it internally?

Is there a reasonable way to regression-test this on Linux? If not and you have verified that it fixes the problem on Windows, I'll approve.

[john-h-kastner]

This would test the reference problem, but unfortunately exposes a rewriting bug. It looks like __atribute__ breaks rewriting.

void foo( const char *fmt, __builtin_ms_va_list argp);
__attribute__((ms_abi)) void bar ( const char *fmt, ...)  {
  __builtin_ms_va_list argp;
  __builtin_ms_va_start(argp, fmt);
  foo(fmt, argp);
  __builtin_ms_va_end(argp);
}

tests do pass on windows now.

[mattmccutchen-cci]

This would test the reference problem, but unfortunately exposes a rewriting bug. It looks like __atribute__ breaks rewriting.

Thanks for putting this test case together. We can leave #450 open just to add this regression test once the __attribute__ bug is fixed (which we can also file separately).

tests do pass on windows now.

Great!