Add @_effects(readnone) to string-based Regex inits

Sources/_StringProcessing/Regex/AnyRegexOutput.swift

@@ -178,10 +178,12 @@ extension Regex where Output == AnyRegexOutput {
   /// ``init(_:as:)`` initializer instead.
   ///
   /// - Parameter pattern: A string with regular expression syntax.
+  @_effects(readnone)
   public init(_ pattern: String) throws {
     self.init(ast: try parse(pattern, .traditional))
   }
 
+  @_effects(readnone)
   internal init(_ pattern: String, syntax: SyntaxOptions) throws {
     self.init(ast: try parse(pattern, syntax))
   }
@@ -212,6 +214,7 @@ extension Regex {
   /// - Parameters:
   ///   - pattern: A string with regular expression syntax.
   ///   - outputType: The desired type for the output captures.
+  @_effects(readnone)
   public init(
     _ pattern: String,
     as outputType: Output.Type = Output.self

Sources/_StringProcessing/Regex/Core.swift

@@ -103,6 +103,7 @@ public struct Regex<Output>: RegexComponent {
   }
 
   // Compiler interface. Do not change independently.
+  @_effects(readnone)
   @usableFromInline
   init(_regexString pattern: String) {
     self.init(ast: try! parse(pattern, .traditional))