gdb: add type::is_vector / type::set_is_vector

gdb: add type::is_vector / type::set_is_vector

Add the `is_vector` and `set_is_vector` methods on `struct type`, in
order to remove the `TYPE_VECTOR` macro.  In this patch, the macro is
changed to use the getter, so all the call sites of the macro that are
used as a setter are changed to use the setter method directly.  The
next patch will remove the macro completely.

gdb/ChangeLog:

	* gdbtypes.h (struct type) <is_vector, set_is_vector>: New methods.
	(TYPE_VECTOR): Use type::is_vector, change all write call sites to
	use type::set_is_vector.

Change-Id: I415e8d169f058662e0750329bfa4017bea3ca0cb