[CodeCompletion] Be lenient in callee analysis

Accept `getInterfaceType->hasError()` declarations. Even if the part of
the declaration has error, we still have chance to get context info from
the other part of it. For instance:

  func foo(x: Int, y: INt) { }
  foo(x: #^COMPLETE^#

We should resolve 'Int' as the context type even if parameter `y` is an
error type.

lib/IDE/CodeCompletion.cpp

@@ -4993,12 +4993,13 @@ void collectPossibleCalleesByQualifiedLookup(
     Type declaredMemberType = VD->getInterfaceType();
     if (auto *AFD = dyn_cast<AbstractFunctionDecl>(VD))
       if (AFD->getDeclContext()->isTypeContext())
-        declaredMemberType = AFD->getMethodInterfaceType();
+        declaredMemberType =
+            declaredMemberType->castTo<AnyFunctionType>()->getResult();
 
     auto fnType =
         baseTy->getTypeOfMember(DC.getParentModule(), VD, declaredMemberType);
 
-    if (!fnType || fnType->hasError())
+    if (!fnType)
       continue;
     if (auto *AFT = fnType->getAs<AnyFunctionType>()) {
       candidates.emplace_back(AFT, VD);