[clang] [Clang] Fix dependent local class instantiation bugs (PR #134038)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > This once again looks like an attempt at hacking around our lack of delayed lambda body instantiation... And this one has some pretty sizable negatives as far as I can see.  I'm not sure I really like this direction.
> 
> Yeah, admittedly this is another workaround of not having that mechanism. But the bug was a 15 regression, (which means we don't have to rely on that for it to work!) and due to some historic incorrect patches (which I'm fixing here) it ends up becoming hard to get the behavior correct as 14.

Oh, totally understood.  This is just a 'stuck between a rock and a hard place' sort of situation, and I'm pretty uncomfortable here again.

I wouldn't be able to convince you to start work on the delayed lambda body instantiation instead, would I ? :-P

https://github.com/llvm/llvm-project/pull/134038