gdb/dwarf: use gdb::unordered_set for cooked_index_storage::m_reader_hash

Replace an htab with gdb::unordered_set.  I think we could also use the
dwarf2_per_cu pointer itself as the identity, basically have the
functional equivalent of:

  gdb::unordered_map<dwarf2_per_cu *, cutu_reader_up>

But I kept the existing behavior of using dwarf2_per_cu::index as the
identity.

Change-Id: Ief3df9a71ac26ca7c07a7b79ca0c26c9d031c11d
Approved-By: Tom Tromey <tom@tromey.com>