Now that we have ArchetypeBuilder::mapTypeOutOfContext(), we can
delete some tricky hand-crafted logic for getting the depth and
index of archetypes.
Notice that the depth of an archetype is now the same as generic
parameters, where depth 0 is the outermost generic context.
Previously it was backwards.
Mostly NFC, except that a few IDE crashers are now fixed because
of asserts firing in removed code, and also the change to depth
mangling (which I think makes sense, and it matches what's written
in docs/ABI.rst).
