[Sema/SILGen] Support for fallthrough into cases with pattern variables.

[gregomni]

This is the simplest / most-restrictive version of this feature. The previous case that we're falling from must have a superset of the bindings of the case we're falling into and all the to-case bindings must match name and type exactly the corresponding from-case binding.

Fallthrough error diagnoses changed for these new rules, Sema changed to check them, SILGen extended to use the same phi argument passing between case blocks for fallthrough that it was already using for multiple patterns. Finally, VarDeclUsage checker changed so that a binding falling through to a next case counts as a read of that decl.

Tests for all the above, including separate SILGen testing for fallthrough of address-only types that @slavapestov recently fixed for multiple patterns.

[gregomni]

@swift-ci Please smoke test.

[gregomni]

@slavapestov Would you like to review or shall I merge? (Primarily my hesitation is just being unsure of what process this sort of enhancement ought to take as it's somewhere in between just a bug fix and not yet an evolution-discussion-requiring feature.)

[jckarter]

@slavapestov's out right now, but this LGTM. Thanks Greg!

[gregomni]

Thanks @jckarter!