fix DIP25 compilation errors

Author: WalterBright

spec/function.dd

@@ -595,7 +595,7 @@ $(H2 $(LNAME2 optional-parenthesis, Optional Parentheses))
     ---
     struct S {
         int function() callfp() { return &numfp; }
-        int delegate() calldg() { return &numdg; }
+        int delegate() calldg() return { return &numdg; }
         int numdg() { return 6; }
     }
 

spec/struct.dd

@@ -1471,7 +1471,7 @@ $(H2 $(LEGACY_LNAME2 AssignOverload, assign-overload, Identity Assignment Overlo
             int[] buf;
             int a;
 
-            ref S opAssign(ref const S s)
+            ref S opAssign(ref const S s) return
             {
                 a = s.a;
                 return this;