gdb: add type::index_type / type::set_index_type

gdb: add type::index_type / type::set_index_type

Add the `index_type` and `set_index_type` methods on `struct type`, in
order to remove the `TYPE_INDEX_TYPE` macro.  In this patch, the
`TYPE_INDEX_TYPE` macro is changed to use `type::index_type`, so all the
call sites that are used to set the type's index type are changed to use
`type::set_index_type`.  The next patch will remove `TYPE_INDEX_TYPE`
completely.

gdb/ChangeLog:

	* gdbtypes.h (struct type) <index_type, set_index_type>: New
	methods.
	(TYPE_INDEX_TYPE): Use type::index_type.
	* gdbtypes.c (create_array_type_with_stride): Likewise.

Change-Id: I93bdca9de9f3e143d2ccea59310c63745315e18d