Previously we did some of this in decl_name_stack and some of it in the
callers of decl_name_stack. Factor out a single place to pop a name and
its optional following parameters.
Part of making this behavior consistent is that we now track whether an
implicit parameter list was present or absent rather than mapping an
absent list to `InstBlockId::Empty`. This improves our redeclaration
checking and the precision of some diagnostics.
