Introduce and use operation::type_p

There's currently code in gdb that checks if an expression evaluates
to a type.  In some spots this is done by comparing the opcode against
OP_TYPE, but other spots more correctly also compare with OP_TYPEOF
and OP_DECLTYPE.

This patch cleans up this area, replacing opcode-checking with a new
method on 'operation'.

Generally, checking the opcode should be considered deprecated,
although it's unfortunately difficult to get rid of opcodes entirely.

I also took advantage of this change to turn eval_op_type into a
method, removing a bit of indirection.

Reviewed-by: Keith Seitz <keiths@redhat.com>

gdb/eval.c

@@ -1872,18 +1872,21 @@ eval_op_postdec (struct type *expect_type, struct expression *exp,
     }
 }
 
-/* A helper function for OP_TYPE.  */
+namespace expr
+{
 
 struct value *
-eval_op_type (struct type *expect_type, struct expression *exp,
-	      enum noside noside, struct type *type)
+type_operation::evaluate (struct type *expect_type, struct expression *exp,
+			  enum noside noside)
 {
   if (noside == EVAL_AVOID_SIDE_EFFECTS)
-    return value::allocate (type);
+    return value::allocate (std::get<0> (m_storage));
   else
     error (_("Attempt to use a type name as an expression"));
 }
 
+}
+
 /* A helper function for BINOP_ASSIGN_MODIFY.  */
 
 struct value *