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

[via cfe-commits]

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@@ -633,17 +637,24 @@ void LookupResult::resolveKind() {
     getSema().diagnoseEquivalentInternalLinkageDeclarations(
         getNameLoc(), HasNonFunction, EquivalentNonFunctions);
 
+  if ((HasNonFunction && (HasFunction || HasUnresolved)) ||
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PiJoules wrote:

Done. I didn't originally add these lines since they were just moved from a few lines below, but I split up the logic and added comments describing it to the best of my understanding.

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