gdb: add type::code / type::set_code

gdb: add type::code / type::set_code

Add the code and set_code methods on code, in order to remove the
TYPE_CODE macro.  In this patch, the TYPE_CODE macro is changed to use
type::code, so all the call sites that are used to set the type code are
changed to use type::set_code.  The next patch will remove TYPE_CODE
completely.

gdb/ChangeLog:

	* gdbtypes.h (struct type) <code, set_code>: New methods.
	(TYPE_CODE): Use type::code.  Change all call sites used to set
	the code to use type::set_code instead.