[swift/main] Provide literal pattern for convertible regexes

[natecook1000]

This adds an (SPI) API for converting a Regex instance to a regex literal when possible, whether the regex starts as a literal, a run-time-constructed regex, or using the RegexBuilder syntax. The _literalPattern is round-trippable, so a regex created from a _literalPattern will have that same _literalPattern, but it could be different from the original pattern used to create a regex.

This adds another confirmation test, as part of the parse testing. When a parsing test is expected to succeed, and contains supported syntax, this checks that a round-tripped literal pattern maintains its identity.

Cherry pick of #670.

[natecook1000]

@swift-ci Please test

[natecook1000]

@swift-ci Please test

[natecook1000]

@swift-ci Please test Windows platform