gdb/dwarf: make dwarf2_per_bfd::dwo_files a gdb::unordered_set

gdb/dwarf: make dwarf2_per_bfd::dwo_files a gdb::unordered_set

Change the dwarf2_per_bfd::dwo_files htab to a gdb::unordered_set.

No behavior change expected, except maybe the failure case in
lookup_dwo_cutu.  If open_and_init_dwo_file returns nullptr, the
previous code would leave the slot value empty (nullptr).  Is this
legit?  With the new hash table, the only thing we can do really is not
attempt to insert the nullptr value.

Change-Id: I63992f388b1197e696ded4ea483634e8ae67fce4
Approved-By: Tom Tromey <tom@tromey.com>