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

[Vladimir Vereschaka via cfe-commits]

vvereschaka 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? 

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