[clang] [Sema] Fix -Wunreachable-code false negative when operands differ only by implicit casts (PR #149972)

[M. Zeeshan Siddiqui via cfe-commits]

codemzs wrote:

> > @AaronBallman @yronglin Thank you for the review. I don't have write permissions, can one of you please sign-off and push the change for me? Thanks!
> 
> Sure! Could you add a release notes(in clang/docs/ReleaseNotes.rst Bug Fixes in This Version section)? You can follow the style of previous items.

@yronglin Done. 

https://github.com/llvm/llvm-project/pull/149972