gdb/dwarf: make some fields in dwarf2_per_cu private

gdb/dwarf: make some fields in dwarf2_per_cu private

Except for the m_length field, that is already private and has a setter,
make the fields whose values are passed through the constructor private.
The idea is that their values should be constant throughout the life of
the object.  Add some getters and update the callers.

I wasn't sure if making some bitfields public and some private would
change how they are packed, so I checked with "ptype/o", it does not.

Change-Id: I7087bebf69e44d16a36c1dd4d7edf9b8bf085343
Approved-By: Tom Tromey <tom@tromey.com>