Fix this.member completion+quickinfo of overloads

1. Completion after `this.` was empty.
2. Quick info of methods with overloads always chose the first overload,
regardless of whether an argument whose type matched a different overload.

Both have the same cause: the type parameter introduced by
polymorphic `this` is not usable, whereas the original is. In both cases,
the usage is simple -- it doesn't take advantage of the capabilities of
polymorphic `this`.

src/compiler/checker.ts

@@ -7977,6 +7977,11 @@ namespace ts {
                 return true;
             }
             // An instance property must be accessed through an instance of the enclosing class
+            if (type.flags & TypeFlags.ThisType) {
+                // get the original type -- represented as the type constraint of the this type
+                type = getConstraintOfTypeParameter(<TypeParameter>type);
+            }
+            
             // TODO: why is the first part of this check here?
             if (!(getTargetType(type).flags & (TypeFlags.Class | TypeFlags.Interface) && hasBaseType(<InterfaceType>type, enclosingClass))) {
                 error(node, Diagnostics.Property_0_is_protected_and_only_accessible_through_an_instance_of_class_1, symbolToString(prop), typeToString(enclosingClass));

src/services/services.ts

@@ -4105,7 +4105,7 @@ namespace ts {
                         let useConstructSignatures = callExpression.kind === SyntaxKind.NewExpression || callExpression.expression.kind === SyntaxKind.SuperKeyword;
                         let allSignatures = useConstructSignatures ? type.getConstructSignatures() : type.getCallSignatures();
 
-                        if (!contains(allSignatures, signature.target || signature)) {
+                        if (!contains(allSignatures, signature.target) && !contains(allSignatures, signature)) {
                             // Get the first signature if there
                             signature = allSignatures.length ? allSignatures[0] : undefined;
                         }