gdb: add type::length / type::set_length

Add the `length` and `set_length` methods on `struct type`, in order to remove
the `TYPE_LENGTH` 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.

Change-Id: Id1090244f15c9856969b9be5006aefe8d8897ca4