When a conformance can either be synthesized or implied, we tend to prefer
implied. However, if the implied conformance comes from a deserialized
conformance, it will lead to an incomplete conformance and cause a crash.
This is a narrow fix for SR-6105 / rdar://problem/34911378.
There are a number of type witnesses that are introduced by the Clang
importer. Immediately resolve those witnesses *without* going through
the type checker, because there are cases (i.e., deserialized SIL)
where the conformance is created but there is no type checker around.
Fixes rdar://problem/30364905, rdar://problem/31053701,
rdar://problem/31565413 / SR-4565.
