[clang] [clang] allow fetch_{min, max} for atomic pointer types (PR #182699)

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huixie90 wrote:

> I think I'd like to see testcases showing what happens for nullptr.
> 
> Otherwise looks fine, I think.

Thank you for the review. added test point for `nullptr` . There is nothing wrong with `nullptr` per say. 

https://github.com/llvm/llvm-project/pull/182699