[clang] [clang] Fix conflicting declaration error with using_if_exists (PR #167646)

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PiJoules wrote:

> Hi @PiJoules , looks like these changes cause a failure of `Clangd:: protocol.test` test
> 
> * https://lab.llvm.org/buildbot/#/builders/193/builds/15354
> * https://lab.llvm.org/buildbot/#/builders/38/builds/8473
> 
> Reverting this commit fixes the problematic test.
> 
> Would you fix or revert the changes?

Thanks for the notification. Would you mind reverting it? I'm AFK for a bit.

https://github.com/llvm/llvm-project/pull/167646