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

[via cfe-commits]

juj wrote:

> it's a really annoying problem that real-world code takes advantage of laxness

In Unity's case, when in our millions of lines of code some line of code does a bad cast, I find it has not been out of intent to rely on x86 casting laxness, but rather an oversight, or, even more common, a memory corruption when a random memory location is interpreted as a function pointer.

But because the existing diagnostics flags provided by LLVM are absolutely useless to diagnose this issue, and Chrome browser debugger is unable to produce informative error messages in these cast situations, we find fixing these problems to be notoriously difficult to resolve, because we can't begin to tell if it was a random memory corruption-based signature mismatch, or a problem by developer mistake.

In our case these issues would be trivial to fix, if:
a) LLVM had a diagnostic flag to narrow down casts to specifically only the offending ones that don't work in WebAssembly, and
b) Chrome debugger was able to print out "we expected this signature, but we got this signature instead", when attempting to invoke a badly casted function pointer occurs.

I.e. so far as I've seen, in the Unity codebase it has never been a situation of "lazy code intentionally relied on x86 ABI casting behavior." (the majority of devs don't seem to even know what those are)

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