[clang] [clang][ptrauth] Warn about the use of a weak signing schema (PR #157779)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

 > Side thought: Would this warning only trigger on variables where a developer has explicitly added something to the source code to request a non-default signing schema? (In that case, chances are higher that the developer may be able to understand the warning well).

This one I can answer: there's explicit authentication (explicit `__ptrauth`). I *think* in this case it's reasonable to assume enough awareness to understand the reason for the warning.

The more tricky one is something like function pointers: they're implicitly signed so a dev only sees

```cpp
static void(*f)();
```
Or whatever the cursed syntax is :D

In this case there's no existing qualifier, so no real reason to expect there to be any dev awareness. That's why I was thinking that maybe an additional note might be appropriate? I didn't think an explanation should be in the warning itself?

We're currently hoping that overriding this will be rare enough that suppressing the warning is uncommon.

Eventually we may extend this warning to cases where overriding is more likely to be necessary, but I have an idea of how that can be addressed (I just need to make sure it would actually work :D )

https://github.com/llvm/llvm-project/pull/157779