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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

@zyn0217 did you look into, and if so, would you mind including explanations on this part of the issue report:

"Curiously, this bug seems to require a lambda -- if the lambda is replaced by an equivalent struct, the [bug disappears](https://godbolt.org/z/v8Wq4osh8). Also, a direct call to the lambda does not trigger the bug; it must be indirectly called via another constexpr function."

If you didn't look into it, it might be worth as otherwise there could be a simpler fix.

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