This commit adds tracking of the reason a declaration reference is potentially
unavailable to the UnavailableToOptionalExpr AST node and to OverloadChoice. We
will use this reason during SILGen to emit the appropriate run-time check and
during typechecking to provide more helpful diagnostics.
To keep OverloadChoice as small as possible, we encode the reason as an index
into a vector of reasons stored in a given instance of ConstraintSystem (this is
the same approach that Fix takes).
This commit adds Sema/OverloadChoice.cpp (for the parts of OverloadChoice that
now rely on ConstraintSystem) and AST/Availability.h (to bring in
availability-related structures without TypeRefinementContext).
Swift SVN r22377
