Differentiate between .multilineLiteral and .multilineExtendedSyntax

The former should always be set in a multi-line literal, the latter
may be partially unset when extended syntax is disabled in a multi-line
literal.

Author: hamishknight

Sources/_RegexParser/Regex/Parse/LexicalAnalysis.swift

@@ -597,7 +597,7 @@ extension Source {
         }.value
 
         // In multi-line literals, the quote may not span multiple lines.
-        if context.syntax.contains(.multilineExtendedSyntax),
+        if context.syntax.contains(.multilineLiteral),
             contents.spansMultipleLinesInRegexLiteral {
           throw ParseError.quoteMayNotSpanMultipleLines
         }
@@ -836,8 +836,7 @@ extension Source {
           throw ParseError.cannotRemoveSemanticsOptions
         }
         // Extended syntax may not be removed if in multi-line mode.
-        if context.syntax.contains(.multilineExtendedSyntax) &&
-            opt.isAnyExtended {
+        if context.syntax.contains(.multilineLiteral) && opt.isAnyExtended {
           throw ParseError.cannotRemoveExtendedSyntaxInMultilineMode
         }
         removing.append(opt)

Sources/_RegexParser/Regex/Parse/Parse.swift

@@ -317,7 +317,7 @@ extension Parser {
     // engines such as Oniguruma, Java, and ICU do this under (?x). Therefore,
     // treat (?x) and (?xx) as the same option here. If we ever get a strict
     // PCRE mode, we will need to change this to handle that.
-    if !context.syntax.contains(.multilineExtendedSyntax) {
+    if !context.syntax.contains(.multilineLiteral) {
       mapOption(.extendedSyntax, \.isAnyExtended)
     }
   }

Sources/_RegexParser/Regex/Parse/SyntaxOptions.swift

@@ -58,9 +58,14 @@ public struct SyntaxOptions: OptionSet {
   /// `(_: .*)` == `(?:.*)`
   public static var experimentalCaptures: Self { Self(1 << 5) }
 
+  /// The syntax kind of a multi-line literal. This will always be set when
+  /// parsing a multi-line literal. Note this does not imply extended syntax,
+  /// as that may be temporarily disabled while parsing.
+  public static var multilineLiteral: Self { Self(1 << 6) }
+
   /// The default syntax for a multi-line regex literal.
   public static var multilineExtendedSyntax: Self {
-    return [Self(1 << 6), .extendedSyntax]
+    return [.multilineLiteral, .extendedSyntax]
   }
 
   /// `(?n)`