swift234

implement <rdar://problem/16393849> Closures cannot define variable parameters: closure exprs parsing patterns as tuple-types

we were previously lookahead parsing the argument list of a closure 
literal as a tuple-type, when we should be doing so as a tuple-pattern.
This doesn't permit some pattern stuff, such as var/let on arguments.

Implement pattern parsing lookahead and use it.


Swift SVN r15350