[CSApply] Register argument matches for dynamic member lookup subscripts

We missed this before because the argument/parameter types are equal.

Author: xedin

lib/Sema/CSApply.cpp

@@ -3687,6 +3687,8 @@ namespace {
       if (!argExpr)
         return nullptr;
 
+      solution.recordSingleArgMatchingChoice(cs.getConstraintLocator(expr));
+
       // Build an argument list.
       auto *argList =
           ArgumentList::forImplicitSingle(ctx, ctx.Id_dynamicMember, argExpr);