gdb/dwarf: convert dwarf2_per_bfd::signatured_types to a gdb::unordered_set

gdb/dwarf: convert dwarf2_per_bfd::signatured_types to a gdb::unordered_set

I'd like to add these asserts in dwarf2_per_cu_data's constructor:

    gdb_assert (per_bfd != nullptr);
    gdb_assert (section != nullptr);

However, these dummy instances of signatured_type, used as hash table
lookup keys, are in the way:

    signatured_type find_sig_entry (nullptr, nullptr, sect_offset {}, sig);

This motivated me to convert the dwarf2_per_bfd::signatured_types hash
table to a gdb::unordered_set.  This allows finding an entry by simply
passing the signature (an integer) and removes the need to create dummy
signatured_type objects.

There is one small unfortunate pessimization:
lookup_dwo_signatured_type, for instance, does a lookup by signature to
find an existing signatured_type.  If not found, it adds a new one by
calling add_type_unit.  The current code passes down the slot where to
put the new element, avoiding an extra hash when inserting the new
signatured_type.  With the new code, I don't see a way to do that.  This
is probably negligible, but it bugs me.  If we used a map of signature
-> signatured_type, I would see a way, but then it would waste space.

I see one change in behavior, that is not really important IMO.  In
read_comp_units_from_section, if duplicate signature type entries are
detected, the code prior to this patch overwrites the existing
signatured_type in the hash table with the new one.  With this patch,
the existing signatured_type is kept: the call to emplace does not
insert the value if an existing equal value exists.

Other than that, no behavior change expected.

Change-Id: I0bef1b49cc63dbdf4e32fa9d4ea37f83025efc3e
Approved-By: Tom Tromey <tom@tromey.com>