[clang] [clang] Implement -Walloc-size diagnostic option (PR #150028)

[Balazs Benics via cfe-commits]

steakhal wrote:

> [...] (In fact, the static analyzer already has the [unix.Malloc](https://clang.llvm.org/docs/analyzer/checkers.html#unix-malloc-c) checker which reports various bugs related to Malloc. If I recall correctly, zero-sized allocations are already modeled and reported when they are illegal; **while negative-sized allocations are not currently covered but would be easy to handle.)**

I didn't read the discussion but I'm not sure how to interpret this highlighted sentence. `malloc` takes an unsigned parameter, thus it won't be ever negative. What I advocated for a long time to consider the malloc parameter as-if it was `rsize_t` (introduced by C11 Annex K [N1570](https://www.iso-9899.info/n1570.html), which is basically `size_t` except that the most significant bit is never supposed to be set. Like passing a negative value to such API, it would go through a signed->unsigned conversion, thus set the MSB; thus its an effective way of detecting "negative" arguments))
Is it similar to what you have in mind?

https://github.com/llvm/llvm-project/pull/150028