swift234

remove the Failure::IsNotOptional failure mode, as CSDiags does a better job
of providing contextual diagnostics (e.g. producing the warning in 
Constraints/dynamic_lookup.swift).  This drops a specific diagnostic about
force casting the result of as! which was added in the Swift 1.2 timeframe
to explain the change in cast semantics.  Now that as! has been around for
a long time, it is more confusing than helpful.



Swift SVN r31887