[clang] [clang] Make sure the same UsingType is searched and inserted (PR #79182)

[Wei Wang via cfe-commits]

apolloww wrote:

We saw a huge build speed regression from internal codebase when migrating to clang-17 triggered by this issue. The search for the same `UsingType` always ends up with "not found" and the folding set `UsingTypes` contains lots of duplicated notes.  

This is a quick fix I come up with. Feel free to propose a better one because I don't work in clang front end often. If it looks good, I'll try to add a test case.   

https://github.com/llvm/llvm-project/pull/79182