[clang][NFC] Add specificity to compatibility hack

[urnathan]

Summary

Add specific dates and versions to note about source_location handling.

This wording matches how I have marked up other compatibility hacks (eg D100680). In 6 years we'll know it is removable :)

[aaron.ballman]

LGTM, thank you for the extra information!

clang/lib/AST/ExprConstant.cpp

if (!E->getType()->isVoidPointerType()) {
       // actual pointee object is actually a cv2 T.
       bool VoidPtrCastMaybeOK =
           !Result.InvalidBase && !Result.Designator.Invalid &&
           !Result.IsNullPtr &&
           Info.Ctx.hasSameUnqualifiedType(Result.Designator.getType(Info.Ctx),
                                           E->getType()->getPointeeType());
       // 1. We'll allow it in std::allocator::allocate, and anything which that
       //    calls.
-      // 2. We'll allow it in the body of std::source_location:current.  This is
-      //    necessary for libstdc++'s <source_location>, which gave its
-      //    parameter the type void*, and cast from that back to `const __impl*`
-      //    in the body. (Fixed for new versions in gcc.gnu.org/PR104602).
+      // 2. HACK 2022-03-28: Work around an issue with libstdc++'s
+      //    <source_location> header. Fixed in GCC 12 and later (2022-04-??).
+      //    We'll allow it in the body of std::source_location::current.  GCC's
+      //    implementation had a parameter of type `void*`, and casts from
+      //    that back to `const __impl*` in its body.
       if (VoidPtrCastMaybeOK &&
           (Info.getStdAllocatorCaller("allocate") ||
            IsDeclSourceLocationCurrent(Info.CurrentCall->Callee))) {
         // Permitted.
       } else {
         Result.Designator.setInvalid();
         if (SubExpr->getType()->isVoidPointerType())
           CCEDiag(E, diag::note_constexpr_invalid_cast)