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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

There is a lot of activity going on just in `clang/test/AST`, with multiple commits a day.
I think there is a time and place, and the right tradeoffs for all kinds of tests we have. Shall we get others opinions?
Shall we make an RFC so we get the community aligned on this new direction?

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