swift234

Clean up the semantics of 'let' properties in a number of ways:

- We switch to a model where let properties may be "initialized", but never
  reassigned.  Specifically, immutable properties in structs/classes may have
  an init value specified in their declaration (but can then never be reset
  in any init implementation) or not (in which case they must be initialized 
  exactly once on all paths through every init.  This makes a lot more sense 
  for immutability, defines several problems away, and provides a path to
  supporting things like (rdar://16181314)

- We now *never* default initialize an immutable property.  Formerly
  we would default initialize optional let properties to nil, but this
  isn't actually useful, and allows an error of omission with let 
  properties.  

This resolves: <rdar://problem/19035287> let properties should only be initializable, not reassignable
and possibly other radars.





Swift SVN r23779