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
Needed to represent deinitializing a partially-initialized existential in which the value witness table has been populated by alloc_existential but the value has not been initialized yet.
Swift SVN r3565
