Previously when fixing ambiguous lookup diagnostics in cc1b6668c57170cd440d321037ced89d6a61a9cb The change refactored `LookupResult` to split out diagnosing access and ambiguous lookups. The call to `getSema().CheckLookupAccess(...)` should have guarded by a check for isAmbiguous(). This change adds that guard. Fixes: https://github.com/llvm/llvm-project/issues/80435