restructure dwarf3 DIE tracing

restructure dwarf3 DIE tracing
* add a trace_DIE function
* use it to trace a bad DIE
  and to trace all DIEs that are (maybe) read

(due to the "avoid read twice" optimisation, the tracing was not
so easy to read anymore => add an explicit trace_DIE call at the beginning
of read_DIE)

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14050