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

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cmtice wrote:

I seem to have found an issue with this warning.  I have some code roughly like this:

if (var1 < 0)
    return;

var2 = malloc(sizeof(...) * var1).

I'm getting warnings complaining that  the malloc is assigning -8 instead of 8.  The only way it could be -8 would be if var1 was negative. But if var1 is negative the malloc never gets executed (because of the return above). So it's warning in a case that can't happen.

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