[gdb/symtab] Handle self-reference DIE

[gdb/symtab] Handle self-reference DIE

While working on a dwarf assembly test-case I accidentally created the
following pathological dwarf:
...
 <1><be>: Abbrev Number: 3 (DW_TAG_class_type)
    <bf>   DW_AT_name        : c1
    <c2>   DW_AT_specification: <0xbe>
...
and noticed gdb segfaulting during cooked index creating due to running out of
stack.  This is a regression from gdb-12, where gdb just hung.

Fix this by inhibiting the scan_attributes self-recursion for self-references.

The same test-case with -readnow makes gdb hang, so also fix this in
dwarf2_attr and follow_die_ref.

Note that this doesn't fix the same problems for the more complicated case of:
...
 <1><be>: Abbrev Number: 3 (DW_TAG_class_type)
    <bf>   DW_AT_name        : c1
    <c2>   DW_AT_specification: <0xc6>
 <1><c6>: Abbrev Number: 4 (DW_TAG_class_type)
    <c7>   DW_AT_name        : c2
    <ca>   DW_AT_specification: <0xbe>
...
but the approach for deciding whether to fix pathological dwarf cases is as
per PR27981 comment 3:
...
yes if it is cheap/obvious, and no if it is something complicated or expensive.
...
and at this point I'm not sure whether fixing this will fall in the first
category.

Tested on x86_64-linux.

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