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

[Erich Keane 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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erichkeane wrote:

This `if` statement is pretty mind-bending.  Can we extract variables/etc to make this more clear what is going on?  Either way it desperately needs a comment.

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