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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

I took a look at what sort of complexity unittest would entail here. I don't think it's a good compromise complexity wise, it's a lot of boilerplate just to test a few AST nodes are correctly linked.

On the other hand, these AST tests don't look particularly out of place compared to a lot of other AST tests we have.

However, I have seen that there are a lot of other related merging issues, so I will leave the tangentially related tests for another MR.

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