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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

I have rebased on top of the change which reenables GMF ODR checker on the test suite, so I added a new regression test that exercises the false positive ODR violation.

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