Previously when fixing ambiguous lookup diagnostics in
cc1b6668c5 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