Before lookup was requestified, the entire lookup stack would
install semantic members. As this caused cycles in the lookup path, it
was refactored to instead only occur at the TypeChecker::lookup*
entrypoints. Unfortunately, these entrypoints were not kept in sync with
the stack building code in qualified lookup, so a case was missed:
class-bound archetypes. We need to synthesize semantic members for them
as well or we'll non-deterministically fail to find synthesizable
members in incremental mode.
rdar://74174749
