Mark '\O' unsupported

We have decided not to support this for now.

Author: hamishknight

Sources/_RegexParser/Regex/Parse/Sema.swift

@@ -220,14 +220,14 @@ extension RegexValidator {
     _ esc: AST.Atom.EscapedBuiltin, at loc: SourceLocation
   ) {
     switch esc {
-    case .resetStartOfMatch, .singleDataUnit,
+    case .resetStartOfMatch, .singleDataUnit, .trueAnychar,
         // '\N' needs to be emitted using 'emitAny'.
         .notNewline:
       error(.unsupported("'\\\(esc.character)'"), at: loc)
 
     // Character classes.
     case .decimalDigit, .notDecimalDigit, .whitespace, .notWhitespace,
-        .wordCharacter, .notWordCharacter, .graphemeCluster, .trueAnychar,
+        .wordCharacter, .notWordCharacter, .graphemeCluster,
         .horizontalWhitespace, .notHorizontalWhitespace,
         .verticalTab, .notVerticalTab:
       break

Tests/RegexTests/MatchTests.swift

@@ -1786,7 +1786,7 @@ extension RegexTests {
       match: eDecomposed,
       xfail: true
     )
-    firstMatchTest(#"\O"#, input: eComposed, match: eComposed)
+    firstMatchTest(#"\O"#, input: eComposed, match: eComposed, xfail: true)
     firstMatchTest(#"\O"#, input: eDecomposed, match: nil,
               xfail: true)
 

Tests/RegexTests/ParseTests.swift

@@ -806,6 +806,8 @@ extension RegexTests {
     parseTest(#"\M-\C--"#, atom(.keyboardMetaControl("-")), unsupported: true)
     parseTest(#"\M-a"#, atom(.keyboardMeta("a")), unsupported: true)
 
+    parseTest(#"\O"#, escaped(.trueAnychar), unsupported: true)
+
     // MARK: Comments
 
     parseTest(