[clang] [Clang][AST/Sema] Fix lambda function pointer conversion operator immediate escalation (PR #235356)

[Ferdinand Bachmann via cfe-commits]

Ferdi265 wrote:

Hmm, this fix breaks the following code that compiles fine on Clang trunk and GCC, but fails with this PR (and same with `static consteval`):

```c++
int main() {
    auto a = [](auto)consteval{return 3;};
    auto b = ((int(*)(int))a)(0);
}
```

https://github.com/llvm/llvm-project/pull/235356