gdb: ensure has dwarf info before reading DWZ file

gdb: ensure has dwarf info before reading DWZ file

I recent change (e9b738dfbdc "Avoid race when reading dwz file") moved
the call to dwarf2_read_dwz_file from dwarf2_initialize_objfile to
dwarf2_has_info.

Before that patch, dwarf2_initialize_objfile was only called when
dwarf2_has_info returned true, and since that patch it is always called.

When reading a file that has no debug info (.debug_info/.debug_abbrev
sections), but has a .gnu_debugaltlink section, GDB’s behavior is
different.  I can observe this when loading
/lib/x86_64-linux-gnu/libtinfo.so on Ubuntu 22.04 (or while debugging
any program dynamically loading this library).

Before e9b738dfbdc, we had:

    $ ./gdb/gdb -data-directory ./gdb/data-directory -q  /lib/x86_64-linux-gnu/libtinfo.so
    Reading symbols from /lib/x86_64-linux-gnu/libtinfo.so...
    (No debugging symbols found in /lib/x86_64-linux-gnu/libtinfo.so)
    (gdb)

while after we have:

    $ ./gdb/gdb -data-directory ./gdb/data-directory -q  /lib/x86_64-linux-gnu/libtinfo.so
    Reading symbols from /lib/x86_64-linux-gnu/libtinfo.so...

    warning: could not find '.gnu_debugaltlink' file for /usr/lib/x86_64-linux-gnu/libtinfo.so.6.3
    (No debugging symbols found in /lib/x86_64-linux-gnu/libtinfo.so)
    (gdb)

This patch restores the previous behavior of only trying to load the
DWZ file for objfiles when the main part of the debuginfo is present
(i.e. when dwarf2_has_info returns true).  We still make sure that
dwarf2_read_dwz_file is called at most once per objfile.

A consequence of this change is that the per_bfd->dwz_file optional
object can now remain empty (instead of containing a nullptr), so also
this patch also adjusts dwarf2_get_dwz_file to account for this
possibility.  This effectively reverts the changes to
dwarf2_get_dwz_file done by e9b738dfbdc.

Regression tested on x86_64-linux-gnu Ubuntu 22.04.

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