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

[Younan Zhang via cfe-commits]

zyn0217 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, and due to some historic incorrect patches (which I'm fixing here) it ends up becoming hard to get the behavior correct as 14.

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