swift234

Several tangled up changes:
 - Enable 'catch is NSError' and 'catch let e as NSError {' patterns to
   a) work, and b) be considered to be exhaustive catches.  This enables
   people to catch an error and *use* it as an NSError directly, instead
   of having to do boiler-platey cases.  This is particularly important
   for the migrator.

 - Do not warn about non-noop coersion casts (like "_ as NSError" when
   matching an ErrorType), since they provide useful type adjustment to
   the subpattern.  Still warn on noop ones.

 - Simplify CatchStmt::isSyntacticallyExhaustive to use 
   Pattern::isRefutablePattern.  Add a FIXME, because the parser is guiding
   closure "throws" inference before the pattern is type checked, which means
   that it is incorrect (but only in subtle cases).

 - When diagnosing pointless 'as' patterns like:

  switch 4 {
  case _ as Int: break 

say "'as' test is always true" instead of "'is' test is always true".




Swift SVN r28774