swift234

Construct type refinement contexts for #os(...).

This patch adds construction of type refinement contexts for the Then branch of
availability queries and adds diagnostics for ill-formed queries. For the
moment, we do not refine for the Else branch because the version range lattice
is not precise enough to do so helpfully.

This patch does not properly handle availability queries involving application
extension platforms (noted with a FIXME). I will add support for these platforms
in a future commit.


Swift SVN r22165