gdb/dwarf: pass is_debug_types to dwarf2_per_cu constructor, make field private

Make the field private to make it clear it is never meant to change.
Pass its value through the constructor, and add a getter.  The only
place that passes true is the signature_type constructor.

Change-Id: Ifb76bc015bca16696fd66cdf45c048b4ba713479
Approved-By: Tom Tromey <tom@tromey.com>