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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

I just pushed the change on this patch. What I meant is this patch will get, besides aforementioned regression test involving ODR checker failure, a simpler AST test that verifies just the UsingShadowDecl is correctly merged.

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