Add test for backreference before group (#172)
Add test for backreference before group (#172)

A literal can specify a backreference that references a group
before the group is opened. This most frequently shows up with a
repeated alternation - the first part of an alternation can include
a reference to a group in a different alternation, and simply
doesn't match anything until the group has matched and captured
a portion of the input.
apple/swift-experimental-string-processing,apple/swift-experimental-string-processing,apple/swift-experimental-string-processing