[Sema/Index] Resolve #keyPath components so they can be indexed

Unlike \keypath expressions, only the property components of #keypath
expressions were being resolved, so index wouldn't pick up references for their
qualifying types.

Also fixes a code completion bug where it was reporting members from the Swift
rather than ObjC side of bridged types.

Resolves rdar://problem/61573935

lib/Sema/CSSimplify.cpp

@@ -8051,6 +8051,9 @@ ConstraintSystem::simplifyKeyPathConstraint(
     case KeyPathExpr::Component::Kind::TupleElement:
       llvm_unreachable("not implemented");
       break;
+    case KeyPathExpr::Component::Kind::DictionaryKey:
+      llvm_unreachable("DictionaryKey only valid in #keyPath");
+      break;
     }
   }