gdb: remove TYPE_FIELDS macro

Remove all uses of the `TYPE_FIELDS` macro.  Replace them with either:

1) type::fields, to obtain a pointer to the fields array (same as
   TYPE_FIELDS yields)
2) type::field, a new convenience method that obtains a reference to one
   of the type's field by index.  It is meant to replace

     TYPE_FIELDS (type)[idx]

   with

     type->field (idx)

gdb/ChangeLog:

	* gdbtypes.h (struct type) <field>: New method.
	(TYPE_FIELDS): Remove, replace all uses with either type::fields
	or type::field.

Change-Id: I49fba10114417deb502060c6156aa5f7fc62462f