[clang] [Clang] Fix confusing diagnositcs related to explicit this parameters (PR #100351)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

Thank you for this PR, could please put a little more detail in the summary since that is what will be seen by folks using git log.

https://github.com/llvm/llvm-project/pull/100351