swift234

Add initial support for importing with tighter Objective C pointer types.

Add the ability to store optionality of the ObjC method parameters and return
type in a sidecar. This hardcoded info is then used to import Objective C
object pointer types as either optional or none, instead of implicitly
unwrapped optionals.

The feature is enabled with -import-with-tighter-objc-types=true.

Swift SVN r19048