Index-while-building ends up querying some conformances that aren’t
strictly needed by the type-checker, but which require us to pre-check all
conformances for any extension we have. Long-term, this is wasted effort, but
for now it eliminates a recent regression caused by the request-evaluator,
which (intentionally) tries to perform less work for non-primary files.
Fixes rdar://problem/41392714.
