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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

I have revived this PR after a long time.

The original ODR violation bad diagnostic from the GH issue is gone from main, so I have removed the test for it.
My best guess is that the ODR checking must have gotten weakened somehow.
I really don't have time to dig into it though.

The original bug is still there in main, is self evident from looking at the source code, and can be confirmed bad and that it gets fixed by just looking at the AST dump.

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