[clang] [clang] Implement `__is_pointer_interconvertible_base_of()` (PR #88473)

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https://github.com/Sirraide commented:

Just pointed out some minor things I noticed, otherwise this looks right to me. 

I at least can’t think of any more test cases that wouldn’t just end up being tests for `IsDerivedFrom()` or `isStandardLayout()`, so I think this is enough.

https://github.com/llvm/llvm-project/pull/88473