[clang] [clang] implement current direction of CWG2765 for string literal comparisons in constant evaluation (PR #109208)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I'm happy once SOME level of solution for the index-starting-value is done (`NextStringLiteralVersion=rand()` 🤡).  I think the avoided collisions is worth a touch of effort, but not much  more.  

I think there is some concern from others about `opaque` that should probably be settled on, but I'm happy with whatever all of you come up with, even if it is `opaque`.

https://github.com/llvm/llvm-project/pull/109208