`CharacterClass.newlineSequence` crashes as unsupported

[hamishknight]

For the following:

import RegexBuilder

let r = Regex {
  CharacterClass.newlineSequence
}
_ = try! r.wholeMatch(in: "\n")

Crashes with:

Fatal error: 'try!' expression unexpectedly raised an error: Unsupported: 'Consumer for unconverted(_StringProcessing.DSLTree._AST.Atom(ast: \n))'
  /Users/hamish/src/swift-experimental-string-processing/Sources/_StringProcessing/ConsumerInterface.swift:317

rdar://96330096

[hamishknight]

cc @natecook1000