gdb: add type::num_fields / type::set_num_fields

gdb: add type::num_fields / type::set_num_fields

Add the `num_fields` and `set_num_fields` methods on `struct type`, in
order to remove the `TYPE_NFIELDS` macro.  In this patch, the
`TYPE_NFIELDS` macro is changed to use `type::num_fields`, so all the
call sites that are used to set the number of fields are changed to use
`type::set_num_fields`.  The next patch will remove `TYPE_NFIELDS`
completely.

I think that in the future, we should consider making the interface of
`struct type` better.  For example, right now it's possible for the
number of fields property and the actual number of fields set to be out
of sync.  However, I want to keep the existing behavior in this patch,
just translate from macros to methods.

gdb/ChangeLog:

	* gdbtypes.h (struct type) <num_fields, set_num_fields>: New
	methods.
	(TYPE_NFIELDS): Use type::num_fields.  Change all call sites
	that modify the number of fields to use type::set_num_fields
	instead.

Change-Id: I5ad9de5be4097feaf942d111077434bf91d13dc5