[gdb/symtab] Handle self-reference in inherit_abstract_dies

[gdb/symtab] Handle self-reference in inherit_abstract_dies

Building gdb with gcc 7.5.0 and -flto -O2 -flto-partition=one generates a
self-referencing DIE:
...
 <2><91dace>: Abbrev Number: 405 (DW_TAG_label)
    <91dad0>   DW_AT_abstract_origin: <0x91dace>
...

When encountering the self-reference DIE in inherit_abstract_dies we loop
following the abstract origin, effectively hanging gdb.

Fix this by handling self-referencing DIEs in the loop in
inherit_abstract_dies.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR symtab/30799
https://sourceware.org/bugzilla/show_bug.cgi?id=30799