[PR 31727] Recognize -2 as a tombstone value in .debug_line

[Dmitry.Neverov <dmitry.neverov@jetbrains.com>]

I ran tests on linux/x64, got the following failures:

FAIL: gdb.base/branch-to-self.exp: single-step: si
FAIL: gdb.base/empty-host-env-vars.exp: env_var_name=HOME: show index-cache directory
FAIL: gdb.python/tui-window.exp: Window was updated
FAIL: gdb.threads/clone-attach-detach.exp: bg attach 1: attach (timeout)
FAIL: gdb.threads/step-over-thread-exit.exp: step_over_mode=displaced: non-stop=on: target-non-stop=on: schedlock=off: cmd=continue: ns_stop_all=0: iter 38: continue (timeout)
FAIL: gdb.tui/empty.exp: src: 90x40: box 1 (ll corner is  , not +)
FAIL: gdb.tui/empty.exp: src-regs: 90x40: box 1 (ll corner is |, not +)
FAIL: gdb.tui/empty.exp: src-regs: 90x40: box 2 (ul corner is |, not +)
FAIL: gdb.tui/empty.exp: asm: 90x40: box 1 (ll corner is  , not +)
FAIL: gdb.tui/empty.exp: asm-regs: 90x40: box 1 (ll corner is |, not +)
FAIL: gdb.tui/empty.exp: asm-regs: 90x40: box 2 (ul corner is |, not +)
FAIL: gdb.tui/empty.exp: split: 90x40: box 1 (ll corner is |, not +)
FAIL: gdb.tui/empty.exp: split: 90x40: box 2 (ul corner is |, not +)
FAIL: gdb.tui/empty.exp: split-regs: 90x40: box 1 (ll corner is |, not +)
FAIL: gdb.tui/empty.exp: split-regs: 90x40: box 2 (ul corner is |, not +)
FAIL: gdb.tui/resize-2.exp: curses width 90
FAIL: gdb.tui/resize-2.exp: gdb width 90
FAIL: gdb.tui/resize-2.exp: prompt redisplays after first resize (timeout)
FAIL: gdb.tui/resize-2.exp: prompt redisplays after second resize (timeout)
FAIL: gdb.tui/resize-2.exp: curses width after resize with TUI disabled
FAIL: gdb.tui/resize-one-line.exp: after resizing: src window shows main
FAIL: gdb.tui/resize.exp: source box after resize (ll corner is  , not +)
FAIL: gdb.tui/resize.exp: lines=2: has prompt
FAIL: gdb.tui/resize.exp: lines=1: has prompt

They seem to be unrelated to the change.

I also checked manually that the patch fixes the assertion in the
project where it was triggered 
(https://sourceware.org/bugzilla/show_bug.cgi?id=31727).

--
Dmitry