Parameterize conformance lookup on whether "missing" conformances are allowed.

Many clients of the conformance lookup operations would prefer to get
an invalid conformance (== there is no conformance) rather than a
missing conformance. Parameterize the conformance lookup operations so
that most callers won't see missing conformances, by filtering them
out at the end. Opt-in those callers that do want to see missing
conformances so they can be diagnosed.

lib/Sema/CSSimplify.cpp

@@ -6225,7 +6225,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
   case ConstraintKind::LiteralConformsTo: {
     // Check whether this type conforms to the protocol.
     auto conformance = DC->getParentModule()->lookupConformance(
-        type, protocol);
+        type, protocol, /*allowMissing=*/true);
     if (conformance) {
       return recordConformance(conformance);
     }