swift234

Treat potentially unavailable global variable references as optional.

This patch adds the ability (-enable-experimental-unavailable-as-optional) to
treat potentially unavailable declarations as if they had optional types. For
the moment, this is only implemented for global variables.

The high-level approach is to (1) record the potential unavailability of a
declaration reference in the overload choice during constraint generation; (2)
treat the declaration as if it had an optional type during overload resolution
(this is similar to how optional protocol members are treated); and (3) add an
implicit conversion (UnavailableToOptionalExpr) during constraint application
to represent the run-time availability check and optional injection.

This patch does not implement SILGen for UnavailableToOptionalExpr.


Swift SVN r22245