gdb/dwarf: rename comp_unit_die to top_level_die

The name "comp_unit_die" is a bit misleading, because it can also
represent a type unit (DW_TAG_type_unit).  I think that "top_level_die"
is clear.

Change-Id: Ibaac99897f0ac7499f0f82caeed3385e1e6ee870
Approved-By: Tom Tromey <tom@tromey.com>
3 files changed