swift234

When a generic type has dependent IR and thus requires each
specialization to be separately lowered in IRGen, use the mangling
of the specialized type as the name of the llvm::StructType instead
of the base, unspecialized type.

This tends to produce fewer collisions between IR type names.
LLVM does unique the names on its own, so that's not strictly
necessary, but it's still a good idea because it makes the test
output more reliable and somewhat easier to read (modulo the
impact of bigger type names).  Collisions will still occur if
the type is specialized at an archetype, since in this case we
will fall back on the unspecialized type.