[clang] [analyzer] Use AllocaRegion in MallocChecker (PR #72402)

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

I think the original `alloca(0)` warning message might be clearer/easier to understand. While it might have platform or compiler dependent meaning, those behaviors are non-portable, so I think it is undesirable in most cases and people probably want to be notified about it. Regarding binding the return value outside of `evalCall`, I think that could be addressed in a separate PR. This one does not make the current situation any worse. But the very least we should add a FIXME now (and potentially open a ticket), to reduce the chances of this getting forgotten.

https://github.com/llvm/llvm-project/pull/72402