[clang] [clang] Fix crashes when passing VLA to va_arg (PR #119563)

[Robear Selwans via cfe-commits]

mo7sen wrote:

> While you're here, maybe look at emitting an undefined-behavior warning for this construct? A VLA is never compatible with a function argument: if you try to write an array in an function type, it gets promoted to a pointer. So this construct is guaranteed to produce broken results (which is why nobody has tripped over this before).

Assuming the actual argument is a struct and the `va_arg(args, uint8_t[sizeof(struct)])` is just to get the struct bytes, would that still be considered undefined behaviour?

https://github.com/llvm/llvm-project/pull/119563