swift234

SILGen: Handle tuple explosions systematically.

Introduce an 'RValue' object in SILGen that behaves like a simplified version of IRGen's 'Explosion' object, holding a destructured tuple in a naturally destructured state. This cleans up a bunch of ad-hoc scattered code that destructured tuples in various places. Update SILGenFunction so that the expression visitors all pass around RValues, and update CallEmission to handle argument clauses as RValues instead of a pre-destructured string of arguments. Implement "emit into" optimization of TupleExpr and ScalarToTupleExpr, now that that's easy to do.

Swift SVN r4667