[AST/Sema] Switch protocol requirement inference to use getABIMembers()

- `getAssociatedTypeMembers()`;
- inference of defaults for associated types;
- `ConformanceChecker` methods that iterate over type and
   value requirements.

lib/AST/Decl.cpp

@@ -5533,7 +5533,7 @@ ProtocolDecl::getAssociatedTypeMembers() const {
     contextData->loader->loadAssociatedTypes(
         this, contextData->associatedTypesData, result);
   } else {
-    for (auto member : getMembers()) {
+    for (auto member : getABIMembers()) {
       if (auto ATD = dyn_cast<AssociatedTypeDecl>(member)) {
         result.push_back(ATD);
       }