[clang] [clang] fix merging of UsingShadowDecl (PR #80245)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

Hmm I have been doing user branches for a while, this saves me time in case I want to split the patch off in a separate MR.
It's already the case that this is split into test adding + fixing commits.

In any case, the user branch will be deleted as soon as this is merged.

Can we go ahead, and clarify the rules for the next time, since this is such a small patch and windows CI takes 3 hours?

https://github.com/llvm/llvm-project/pull/80245