[clang] [Clang] Diagnose fixed point arithmetic with `_BitInt` and `overflow` behavior types (PR #221134)

[Akash Manna via cfe-commits]

akash-manna-sky wrote:

> There was a duplicate issue: #196948
> 
> and a fix was landed for that that already fixed this issue: #199912

You're right that #199912 makes the exact reproducer error out cleanly now. But #199912 only special-cased `_BitInt`; the underlying assumption in `GetFixedPointRank` that both operands are `BuiltinType` was left unguarded. Overflow behavior types are arithmetic, aren't `BuiltinType`, and are dispatched *after* the fixed point branch, so trunk still crashes today on

```c
void g(_Fract f, int __ob_wrap w) { (void)(f + w); }
```
@PiJoules @AaronBallman , If everything is fine and do not have any further comments, please merge it for me!

https://github.com/llvm/llvm-project/pull/221134