[AST] Make NormalProtocolConformance::forEachTypeWitness more robust.

If we cannot resolve a type witness, don't try.

Author: DougGregor

include/swift/AST/ProtocolConformance.h

@@ -181,6 +181,10 @@ class alignas(1 << DeclAlignInBits) ProtocolConformance {
       if (!assocTypeReq || req->isInvalid())
         continue;
 
+      // If we don't have and cannot resolve witnesses, skip it.
+      if (!resolver && !hasTypeWitness(assocTypeReq))
+        continue;
+
       const auto &TWInfo = getTypeWitnessSubstAndDecl(assocTypeReq, resolver);
       if (f(assocTypeReq, TWInfo.first, TWInfo.second))
         return true;