swift234

Add ScalarToTupleExpr to represent an implicit conversion from a scalar to a tuple, with or without default arguments.  Make Sema and IRGen work with them.

I'm not completely sure this is the representation we want, but it isn't much code to rip out if we decide to represent it some other way.

While I'm in the area, also fix a case where tuple->tuple conversion wasn't working.



Swift SVN r1748