Don't trap when passing non-POD arguments to variadic functions in MS-compatibility mode

Clang warns (treated as error by default, but still ignored in system headers)
when passing non-POD arguments to variadic functions, and generates a trap
instruction to crash the program if that code is ever run.

Unfortunately, MSVC happily generates code for such calls without a warning,
and there is code in system headers that use it.

This makes Clang not insert the trap instruction when in -fms-compatibility
mode, while still generating the warning/error message.

Differential Revision: http://reviews.llvm.org/D5492

llvm-svn: 218640
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 2cf25fe..b4f90d3 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -3569,6 +3569,7 @@
       break;
 
     case Sema::VAK_Undefined:
+    case Sema::VAK_MSVCUndefined:
       EmitFormatDiagnostic(
         S.PDiag(diag::warn_non_pod_vararg_with_format_string)
           << S.getLangOpts().CPlusPlus11