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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Before looking into it, I remember the user branch is only allowed for stacked review. If this patch is not the case, let's try to create a new PR.

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