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

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 approved this pull request.

LG. It looks not wise to block this good PR. After all, it is a change 6 lines. Let's go ahead.

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