[clang] [clang][WebAssembly] Handle casted function pointers with different number of arguments (PR #153168)

[Jorge Zapata via cfe-commits]

turran wrote:

> What I would like is a new flag exactly like the current `-Wcast-function-type`, but one that was just narrower in its emitted warnings.
> 
> I.e. currently `-Wcast-function-type` emits warnings for all sorts of casts that are safe with Wasm ABI (like casting between `unsigned int` to `int` in a function arg).. but I'd like to not get warnings for those. And only get warnings for a narrower set of casts that will cause a trap at Wasm runtime. That would help audit through the millions of lines of code that e.g. Unity3D engine compiles through. (`-Wcast-function-type` is too much, and `-Wbad-function-cast` is not enough, as per illustrated in #64526)
> 
> Like was pointed out, of course detecting all scenarios that result to Wasm VM trapping at runtime is not possible (e.g. casting `void(int,int) -> void* -> void(int)` and then attempting to call), but that is besides the point. Because we couldn't detect all such scenarios doesn't mean it wouldn't be extremely useful to diagnose all the scenarios that can reasonably be compile-time diagnosed.

Okay, I've re-read the issue you mentioned. Indeed, it makes sense to provide a diagnosis of such use cases. Maybe as part of another PR? This was more in the direction of a different number of arguments instead of a different type of arguments. 

https://github.com/llvm/llvm-project/pull/153168