Squash -Winconsistent-missing-override that comes up due to CRTP.

Sometimes these methods are overriding, sometimes they're not.
Unfortunately, there's not a very consistent solution, so just disable
the warning in these two .cpp files.

No functionality change.

Swift SVN r25604

lib/IRGen/GenStruct.cpp

@@ -37,6 +37,8 @@
 #include "NonFixedTypeInfo.h"
 #include "StructMetadataLayout.h"
 
+#pragma clang diagnostic ignored "-Winconsistent-missing-override"
+
 using namespace swift;
 using namespace irgen;