Unifies behavior for member access of ABI functions.

test/libsolidity/semanticTests/specialFunctions/abi_functions_member_access.sol

@@ -1,6 +1,9 @@
 contract C {
     function f() public pure {
         abi.encode;
+        abi.encodePacked;
+        abi.encodeWithSelector;
+        abi.encodeWithSignature;
         abi.decode;
     }
 }