Disable -Wformat-extra-args for arguments defined in system headers.

Some of the NSAssert macros in OS X 10.7 are implemented in a way that
adds extra arguments that trigger the -Wformat-extra-args warning.
Earlier versions of clang failed to detect those -Wformat issues, but now
that clang is reporting those problems, we need to quiet them since there's
nothing to be done to fix them.  <rdar://problem/11317765>

I don't know how to write a testcase for this.  Suggestions welcome.
Patch by Ted Kremenek!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@156092 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Sema/SemaChecking.cpp

@@ -1975,9 +1975,12 @@ void CheckFormatHandler::DoneProcessing() {
     signed notCoveredArg = CoveredArgs.find_first();
     if (notCoveredArg >= 0) {
       assert((unsigned)notCoveredArg < NumDataArgs);
-      EmitFormatDiagnostic(S.PDiag(diag::warn_printf_data_arg_not_used),
-                           getDataArg((unsigned) notCoveredArg)->getLocStart(),
-                           /*IsStringLocation*/false, getFormatStringRange());
+      SourceLocation Loc = getDataArg((unsigned) notCoveredArg)->getLocStart();
+      if (!S.getSourceManager().isInSystemMacro(Loc)) {
+        EmitFormatDiagnostic(S.PDiag(diag::warn_printf_data_arg_not_used),
+                             Loc,
+                             /*IsStringLocation*/false, getFormatStringRange());
+      }
     }
   }
 }