gdb/dwarf: rename dwarf2_start_subfile to dwarf2_cu::start_subfile

Rename dwarf2_start_subfile to dwarf2_cu::start_subfile.  This
refactor continues the work started in the previous commit.

There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>
5 files changed