clang: Update comments on another libstdc++ HACK

[urnathan]

Summary

This is another workaround for a libstdc++ problem. Both the g++ bug and libstdc++ use of it are fixed in 4.9.0, but present in the 4.8 series. So we can't remove it yet. I also verified, that when the hack is disabled, that g++ 4.9.0 gives the expected diagnostics about types not matching on the testcase Richard added for this hack.

So updating the comment with dates and version numbers..

[bruno]

Thanks for keeping a good track of these hacks. Nitpick: before landing, use single instead of double spaces before a new sentence in the comments. LGTM.

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

if (DI->getType()->isInstantiationDependentType() ||
     if (!DI) {
       Invalid = true;
       DI = SemaRef.Context.getTrivialTypeSourceInfo(SemaRef.Context.IntTy);
     }
   } else {
     SemaRef.MarkDeclarationsReferencedInType(D->getLocation(), DI->getType());
   }
 
-  // HACK: g++ has a bug where it gets the value kind of ?: wrong.
-  // libstdc++ relies upon this bug in its implementation of common_type.
-  // If we happen to be processing that implementation, fake up the g++ ?:
-  // semantics. See LWG issue 2141 for more information on the bug.
+  // HACK: 2012-10-23 g++ has a bug where it gets the value kind of ?: wrong.
+  // libstdc++ relies upon this bug in its implementation of common_type.  If we
+  // happen to be processing that implementation, fake up the g++ ?:
+  // semantics. See LWG issue 2141 for more information on the bug.  The bugs
+  // are fixed in g++ and libstdc++ 4.9.0 (2014-04-22).
   const DecltypeType *DT = DI->getType()->getAs<DecltypeType>();
   CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(D->getDeclContext());
   if (DT && RD && isa<ConditionalOperator>(DT->getUnderlyingExpr()) &&
       DT->isReferenceType() &&
       RD->getEnclosingNamespaceContext() == SemaRef.getStdNamespace() &&
       RD->getIdentifier() && RD->getIdentifier()->isStr("common_type") &&
       D->getIdentifier() && D->getIdentifier()->isStr("type") &&
       SemaRef.getSourceManager().isInSystemHeader(D->getBeginLoc()))