[Clang][Sema] Fix regression due to missing ambiguity check before attempting...

[Clang][Sema] Fix regression due to missing ambiguity check before attempting access check. (#80730)

Previously when fixing ambiguous lookup diagnostics in
cc1b6668 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