DebugInfo: Refactor/deduplicate various template argument list emission

Streamline template arguments across types, variables, and functions -
for convenient reuse in experiments related to template argument list
reconstitution (not including template argument lists in the "name" of
those entities, and leaving it to debug info consumers to rebuild the
full template name from the semantic descriptions of the argument lists)

But the change seems like a good refactoring/cleanup anyway.

I'd certainly be open to suggestions about how this might be more
streamlined - like is there no generic way to query template argument
lists across the 3 kinds of entities, rather than needing special case
code?