gdb/dwarf: add some logging in dwarf2/read.c

gdb/dwarf: add some logging in dwarf2/read.c

This patch adds some logging that helped me diagnose the problems fixed
later in this series.  I'm thinking that if it helped me now, it could
help somebody else (or myself) in the future, so I might as well add
them for real.

They can happen quite frequently and be noisy, so I used
dwarf_read_debug_printf_v for them, which means they'll only print if
`set debug dwarf-read` is >= 2.

gdb/ChangeLog:

	* dwarf2/read.c (follow_die_offset): Add logging.
	(dwarf2_per_objfile::age_comp_units): Add logging.

Change-Id: I7483c0b05c37bc9710b9b5d40e272935bc010863