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

It doesn't seem like regex instantiation is being hoisted out
of loops, which means that both the parsing (which happens at runtime)
and compilation (which happens on first use) are being lost for
string-based and possibly literal regexes. This may address the
issue.

Author: natecook1000

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))