Provide literal pattern for convertible regexes (#670)

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.

Author: natecook1000

Sources/_RegexParser/Regex/Parse/LexicalAnalysis.swift

@@ -2091,9 +2091,7 @@ extension Parser {
         // multiple scalars. These may be confusable for metacharacters, e.g
         // `[\u{301}]` wouldn't be interpreted as a custom character class due
         // to the combining accent (assuming it is literal, not `\u{...}`).
-        let scalars = char.unicodeScalars
-        if scalars.count > 1 && scalars.first!.isASCII && char != "\r\n" &&
-            !char.isLetter && !char.isNumber {
+        if char.isConfusable {
           p.error(.confusableCharacter(char), at: charLoc.location)
         }
         break

Sources/_RegexParser/Utility/Misc.swift

@@ -32,6 +32,19 @@ extension Character {
     let str = String(self)
     return str._nfcCodeUnits.elementsEqual(str.utf8)
   }
+  
+  /// Whether this character could be confusable with a metacharacter in a
+  /// regex literal.
+  ///
+  /// A "confusable" character is one that starts with a non-alphanumeric ASCII
+  /// character and includes other combining Unicode scalars. For example,
+  /// `"[́"` (aka `"[\u{301}"`) is confusable, since it looks just like the
+  /// `"["` metacharacter, but doesn't parse as one.
+  public var isConfusable: Bool {
+    let scalars = self.unicodeScalars
+    return scalars.count > 1 && scalars.first!.isASCII && self != "\r\n" &&
+      !self.isLetter && !self.isNumber
+  }
 }
 
 extension CustomStringConvertible {

Sources/_StringProcessing/CMakeLists.txt

@@ -76,6 +76,7 @@ add_library(_StringProcessing
   Compiler.swift
   ConsumerInterface.swift
   Executor.swift
+  LiteralPrinter.swift
   MatchingOptions.swift
   PrintAsPattern.swift)
 target_compile_options(_StringProcessing PRIVATE