swift234

Implement type sugar "T?" for Optional<T>.

- New type representation OptionalTypeRepr.
- New sugared type OptionalType.
- New base type SyntaxSugarType, parent of ArraySliceType and OptionalType.
  These two are the same in a lot of ways.
- The form "T[]?" is forbidden, because it makes "Int[4][2]" oddly
  different from "Int[4]?[2]". The type can be spelled "(T[])?" or
  Optional<T[]>.
- Like Slice, "Optional" is just looked up in the current module. This may
  or may not be the desired behavior in the long run.

<rdar://problem/14666783>

Swift SVN r7100