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

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 commented:

IIRC, in https://github.com/llvm/llvm-project/pull/79959, you're saying the patch have the potential to fix https://github.com/llvm/llvm-project/issues/78850? Is it true?

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