swift234

Make trailing closure syntax match the last parameter, always.

Previously, trailing closures would try to match the first parameter
of (possibly optional) function type that didn't seem to have an
argument already, but in practice this broke when there were
parameters with default arguments before the function parameter.

The new rule is far simpler: a trailing closure matches the last
parameter. Fixes rdar://problem/17965209.

Swift SVN r24898