C++: Support x-macros that are #undef'ed in header

[jbj]

This change should address the FP reported at https://discuss.lgtm.com/t/false-positive-in-c-header/2265.

The changed predicates take negligible time on all snapshots I've tried. @geoffw0, you must have had some problematic snapshots when you wrote #313. Can you test on those?

I'll create a change note file for 1.23 in a separate PR.

[geoffw0]

Changes LGTM but yes, I need to find some problematic snapshots and make sure performance is still OK...

[geoffw0]

I tried the original (pre-#313) version of the query on 16 projects, the only ones that took over 60 seconds were:

Chromium - original 594s (218 results), master 556s (218 results), PR 558s (215 results)
Libreoffice - original 201s (219 results), master 189s (219 results), PR 188s (215 results)
Linux (full) - original 283s (676 results), master 282s (676 results), PR 280s (674 results)
UnrealEngine - original exec failed; master exec failed; PR exec failed
(timeout was only set to 600s)

It looks like I've failed to find a problematic snapshot that the first PR solves (it's also possible the modern optimizer copes with all versions of the query equally now). Nevertheless, performance is no worse in my testing, and I think the original issue was caused by exposing the macroName variable in the query itself, which your version does not do. So we should be good.

FWIW, I also investigated some of the lost results and they matched your intended pattern. 👍