dwarflint: Add check_die_tree pass for generic per-DIE checks

dwarflint: Add check_die_tree pass for generic per-DIE checks

- add reg_die_check for registering per-DIE check passes
- turn check_registrar into a template to allow reuse among reg<> and
  reg_die_check<>
- the per-DIE checking magic is in check_die_tree.hh
- hide most of checks.hh into dwarflint.cc.  That has little to do
  with per-DIE checks as such, but was a necessary cleanup