gdb: add type::fields / type::set_fields

gdb: add type::fields / type::set_fields

Add the `fields` and `set_fields` methods on `struct type`, in order to
remove the `TYPE_FIELDS` macro.  In this patch, the `TYPE_FIELDS` macro
is changed to the `type::fields`, so all the call sites that use it to
set the fields array are changed to use `type::set_fields`.  The next
patch will remove `TYPE_FIELDS` entirely.

gdb/ChangeLog:

	* gdbtypes.h (struct type) <fields, set_fields>: New methods.
	(TYPE_FIELDS): Use type::fields.  Change all call sites that
	modify the propery to use type::set_fields instead.

Change-Id: I05174ce68f2ce3fccdf5d8b469ff141f14886b33