gdb/dwarf: use gdb::unordered_set for seen_names

gdb/dwarf: use gdb::unordered_set for seen_names

Direct replacement of an htab with a gdb::unordered_set.

Using a large test program, I see a small but consistent performance
improvement.  The "file" command time goes on average from 7.88 to 7.73
seconds (~2%).  To give a rough estimate of the scale of the test
program, the 8 seen_names hash tables (one for each worker thread) had
between 173846 and 866961 entries.

Change-Id: I0157cbd04bb55338bb1fcefd2690aeef52fe3afe
Approved-By: Tom Tromey <tom@tromey.com>