swift234

[stdlib] Trap invalid Ranges of Comparable Indexes

When the Index type of a Range is Comparable, we can reject invalid
ranges at their formation.

It might be worth injecting some checkability concept into ForwardIndex
so that even in generic contexts where Comparability is unknown, we can
do this same check in the Range constructor.

Also, remove pattern matching support for Ranges of RandomAccessIndex,
as that is covered by Interval.

Fixes <rdar://problem/16254937> (#Seed 4: Ranges with negative strides
are broken) to the extent possible (it's still possible to form an
invalid Range of indices that are not Comparable)

Fixes <rdar://problem/17164391> (Swift: Using ranges in for in for
counting down, causes the loop to go inifinely)

Fixes <rdar://problem/17580871> (Swift: Closed range with negative
endpoint excludes that endpoint)

Swift SVN r19903