Fix IRGen to not use outlining for types with any kind of local archetype.
Previously it was testing for opened existentials specifically.
We should really teach outlining to handle local archetypes properly.
We'd have to build a generic signature for the lowered type, and that
probably means also adding requirements that are relevant to value
operations, but it would mean outlining would benefit all types, and
it would let us avoid bundling in unnecessary information from the
enclosing generic environment.
A minor side-effect of this is that we no longer bind names to
opened element type values. The names were things like \tau_1_0,
which is not very useful, especially in LLVM IR where \tau is
printed with two UTF-8 escapes.