[clang] [Clang] Don't give up on an unsuccessful function instantiation (PR #126723)

[via cfe-commits]

https://github.com/cor3ntin commented:

This is missing a changelog entry.

I wonder if `AtEndOfTU` should be a bool in Sema instead (that would replace all existing uses of `AtEndOfTU` - it would simplify this patch and a few other functions. I don't think we need to do it in this patch though.

The general direction looks good

https://github.com/llvm/llvm-project/pull/126723