Fix r217275 to work without the need for standard headers being included

It seems (I guess) in ObjC that va_list is provided without the need for
inclusions. I verified that with this change the test still crashes in
the absence of the fix committed in r217275.

llvm-svn: 217290

clang/test/Modules/Inputs/va_list/module.modulemap

@@ -1,5 +1,2 @@
-module stdarg [system] {
-  header "stdarg.h" // note: supplied by the compiler
-}
 module va_list_a { header "va_list_a.h" }
 module va_list_b { header "va_list_b.h" }

clang/test/Modules/Inputs/va_list/va_list_a.h

@@ -1,2 +1 @@
-@import stdarg;
 int  vprintf(const char * __restrict, va_list);