Fix compatibility for __builtin_stdarg_start

[joerg]

Summary

The __builtin_stdarg_start is the legacy spelling of __builtin_va_start. It should behave exactly the same, but for the last 9 years it would behave subtly different for diagnostics. Follow the change from 29ad95b23217 to require custom type checking.

[joerg]

Committed as 0b999f76575f0196d3cd01c0781b2513b0a1af15 without link.

clang/include/clang/Basic/Builtins.def

 // Random GCC builtins
 BUILTIN(__builtin_constant_p, "i.", "nctu")
 BUILTIN(__builtin_classify_type, "i.", "nctu")
 BUILTIN(__builtin___CFStringMakeConstantString, "FC*cC*", "nc")
 BUILTIN(__builtin___NSStringMakeConstantString, "FC*cC*", "nc")
 BUILTIN(__builtin_va_start, "vA.", "nt")
 BUILTIN(__builtin_va_end, "vA", "n")
 BUILTIN(__builtin_va_copy, "vAA", "n")
-BUILTIN(__builtin_stdarg_start, "vA.", "n")
+BUILTIN(__builtin_stdarg_start, "vA.", "nt")
 BUILTIN(__builtin_assume_aligned, "v*vC*z.", "nc")
 BUILTIN(__builtin_bcmp, "ivC*vC*z", "Fn")
 BUILTIN(__builtin_bcopy, "vv*v*z", "n")
 BUILTIN(__builtin_bzero, "vv*z", "nF")
 BUILTIN(__builtin_fprintf, "iP*cC*.", "Fp:1:")
 BUILTIN(__builtin_memchr, "v*vC*iz", "nF")
 BUILTIN(__builtin_memcmp, "ivC*vC*z", "nF")
 BUILTIN(__builtin_memcpy, "v*v*vC*z", "nF")

clang/test/SemaCXX/vararg-non-pod.cpp

 // rdar://7985267 - Shouldn't warn, doesn't actually use __builtin_va_start is
 // magic.
 
 void t6(Foo somearg, ... ) {
   __builtin_va_list list;
   __builtin_va_start(list, somearg);
 }
 
+// __builtin_stdarg_start is a compatibility alias for __builtin_va_start,
+// it should behave the same
+void t6b(Foo somearg, ... ) {
+  __builtin_va_list list;
+  __builtin_stdarg_start(list, somearg); // second argument to 'va_start' is not the last named parameter [-Wvarargs]
+}
+
 void t7(int n, ...) {
   __builtin_va_list list;
   __builtin_va_start(list, n);
   (void)__builtin_va_arg(list, C); // expected-warning{{second argument to 'va_arg' is of non-POD type 'C'}}
   __builtin_va_end(list);
 }
 
 struct Abstract {