This modifies the logic introduced in D82828 to address the case that
was brought up in PR47150.
Currently, we use tombstone value for all discarded references in
.debug_line section, but we rewrite ICF references in other debug
sections. This introduces inconsistency between .debug_line and
.debug_info data which is a problem for consumers such as GSYM.
This change modifies the behavior to be more consistent: we always
rewrite the reference for ICF folded case across all debug sections,
and we always use the tombstone value for all other cases.