-fms-extensions: Add __va_start builtin, which is used for x64

The main difference between __va_start and __builtin_va_start is that
the address of the va_list has already been taken, and the va_list is
always a char*.

__va_end and __va_arg are not needed.

llvm-svn: 204821
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 0285231..f0e93a7 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -142,6 +142,7 @@
     break;
   case Builtin::BI__builtin_stdarg_start:
   case Builtin::BI__builtin_va_start:
+  case Builtin::BI__va_start:
     if (SemaBuiltinVAStart(TheCall))
       return ExprError();
     break;