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/expression.h

@@ -147,6 +147,11 @@ public:
   virtual bool uses_objfile (struct objfile *objfile) const
   { return false; }
 
+  /* Some expression nodes represent a type, not a value.  This method
+     should be overridden to return 'true' in these situations.  */
+  virtual bool type_p () const
+  { return false; }
+
   /* Generate agent expression bytecodes for this operation.  */
   void generate_ax (struct expression *exp, struct agent_expr *ax,
 		    struct axs_value *value,
@@ -215,6 +220,11 @@ struct expression
     op->dump (stream, 0);
   }
 
+  /* Call the type_p method on the outermost sub-expression of this
+     expression, and return the result.  */
+  bool type_p () const
+  { return op->type_p (); }
+
   /* Return true if this expression uses OBJFILE (and will become
      dangling when OBJFILE is unloaded), otherwise return false.
      OBJFILE must not be a separate debug info file.  */