SILGen: Avoid crashing for invalid conformances.

Force resolution of value witnesses and check the conformance for validity
before proceeding to witness table emission in SILGen to avoid crashing because
of unexpected errors in the AST.

Resolves rdar://123027739

lib/Sema/CodeSynthesis.cpp

@@ -1401,9 +1401,7 @@ ResolveImplicitMemberRequest::evaluate(Evaluator &evaluator,
       // FIXME: This should be more fine-grained to avoid having to check
       // for a cycle here.
       if (!evaluator.hasActiveRequest(ResolveValueWitnessesRequest{conformance})) {
-        evaluateOrDefault(evaluator,
-                          ResolveValueWitnessesRequest{conformance},
-                          evaluator::SideEffect());
+        conformance->resolveValueWitnesses();
       }
     }