DWARFDie: don't try to compute a full template name for a template parameter packs

Otherwise these look a lot like actual templates (they have a name and
they have template parameters) but they don't participate in naming
(this doesn't come up in practice because a template parameter pack DIE
is never referenced from another DIE (so we don't do full name
rebuilding for it) or the subject of simplified template name rebuilding
(never has the _STN prefix)) - it could be tested with some hand crafted
DWARF but doesn't seem important/useful to do so.

This change is just for performance - to avoid trying to parse more
DIEs, etc, when it's not needed when computing the name in the DWARF
verifier.