[gdb/symtab] Fix parent map dump

Before the fix for PR symtab/32225, the parent map dump showed a mapping from
section offsets to cooked index entries:
...
  0x0000000000000035 0x3ba9560 (0x34: sp1::A)
...
but now that's no longer the case:
...
  0x00000000406f5405 0x410a04d0 (0x34: sp1::A)
...

Fix this by extending the annotation somewhat, such that we get:
...
map start:
  0x0000000012c52405 0x135fd550
	(section: .debug_info, offset: 0x35) -> (0x34: sp1::A)
...

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32225