swift234

Start peering through the fog of ClosureExprs, using ambiguously typed subexprs to 
diagnose problems inside of them instead of punting on them completely.

This leads to substantially better error messages in many cases, fixing:
 <rdar://problem/19870975> Incorrect diagnostic for failed member lookups within closures passed as arguments ("(_) -> _")
 <rdar://problem/21883806> Bogus "'_' can only appear in a pattern or on the left side of an assignment" is back
 <rdar://problem/20712541> QoI: Int/UInt mismatch produces useless error inside a block

and possibly others.  We are not yet capitalizing on available type information we do
have about closure exprs, so there are some cases where we produce
  "error: type of expression is ambiguous without more context"
when this isn't strictly true, but this is still a huge step forward.



Swift SVN r30547