This reverts commit 25985cb764. For now,
we're trying to avoid spurious non-structural changes to the mangling,
so that the /old/ mangling doesn't appear to change. That doesn't mean
no changes at all, but we can save this one for later.
Ensure they get emitted at the end of the job by the dispatcher, and
also use a proper mangling and shared linkage for these symbols so
that if multiple threads emit the same descriptor it gets merged.
The new tests attempt to exercise these scenarios.
Fixes <rdar://problem/27906876>.
