swift234

Sema: Teach availability checking about require/else.

We now introduce a TypeRefinementContext for the fallthrough branch of a require/else
statement that continues until the end of the BraceStmt that contains the RequireStmt. The
body of  the else is checked in the context that contains the RequireStmt.

This enables availability checking with early return:

require #available(iOS 8.0, *) else { return }

Swift SVN r28113