DWARFVerifier: Check section-relative references at the end of the section

This ensures that debug_types references aren't looked for in
debug_info section.

Behavior is still going to be questionable in an unlinked object file -
since cross-cu references could refer to symbols in another .debug_info
(or, in theory, .debug_types) chunk - but if a producer only uses
ref_addr to refer to things within the same .debug_info chunk in an
object file (eg: whole program optimization/LTO - producing two CUs into
a single .debug_info section in an object file - the ref_addrs there
could be resolved relative to that .debug_info chunk, not needing to
consider comdat  (DWARFv5 type units or other creatures) chunks of
.debug_info, etc)