Add delimiter parsing support to Regex(_regexString:version:)

Add a new overload of the compiler intrinsic initializer for Regex initialization that allows for a versioned argument, and use it to start parsing a string with delimiters, which allows the syntax options to be picked up at runtime.

Author: hamishknight

Sources/_StringProcessing/RegexDSL/Core.swift

@@ -47,6 +47,15 @@ public struct Regex<Capture>: RegexProtocol {
  self.init(ast: try! parse(pattern, .traditional))
  }
 
+ // Compiler interface. Do not change independently.
+ @usableFromInline
+ init(_regexString pattern: String, version: Int) {
+ assert(version == currentRegexLiteralFormatVersion)
+ // The version argument is passed by the compiler using the value defined
+ // in libswiftParseRegexLiteral.
+ self.init(ast: try! parseWithDelimiters(pattern))
+ }
+
  public init<Content: RegexProtocol>(
  _ content: Content
  ) where Content.Capture == Capture {