swift234

[Constraint solver] Don't attempt obviously-wrong tuple-to-tuple conversions when scalar-to-tuple might work.

When we're trying to convert one tuple to another, and we hit a
problem that doesn't require us to actually perform more type
comparisons (mismatched labels, not enough elements, etc.), fall back
to attempt a scalar-to-tuple conversion when the target tuple can be
initialized by a scalar.

This is *not* a complete solution to this problem. It handles the
obvious cases, but for the non-obvious cases we'll have to pursue both
paths separately to see which one pans out.

Nonetheless, this fixes most of <rdar://problem/11293232>, as well as
<rdar://problem/13820721> and <rdar://problem/13394261>.


Swift SVN r5295