Add @retroactive to conformances that need it. (#714)

SE-0364 requires that retroactive conformance be marked with `@retroactive` to
suppress warnings about the dangers of conforming types you don't own to
protocols you don't own. The `RegexBuilder` module declares conformances to
`RegexComponent` (from the `Regex` module) on several types from the standard
library, like `String`. Since `RegexBuilder` declares neither the protocol nor
the conforming type, these conformances are technically retroactive even though
the modules involved are all developed and distributed together and can be
trusted to organize the conformances this way safely.

The compiler warnings about these conformances are quite spammy; they get
emitted every time someone builds the Swift compiler, so I'd like to suppress
them.

Co-authored-by: Allan Shortlidge <tshortli@gmail.com>

Author: natecook1000

Sources/RegexBuilder/DSL.swift

@@ -32,21 +32,21 @@ internal protocol _BuiltinRegexComponent: RegexComponent {
 // MARK: - Primitive regex components
 
 @available(SwiftStdlib 5.7, *)
-extension String: RegexComponent {
+extension String: @retroactive RegexComponent {
   public typealias Output = Substring
 
   public var regex: Regex<Output> { .init(verbatim: self) }
 }
 
 @available(SwiftStdlib 5.7, *)
-extension Substring: RegexComponent {
+extension Substring: @retroactive RegexComponent {
   public typealias Output = Substring
 
   public var regex: Regex<Output> { String(self).regex }
 }
 
 @available(SwiftStdlib 5.7, *)
-extension Character: RegexComponent {
+extension Character: @retroactive RegexComponent {
   public typealias Output = Substring
 
   public var regex: Regex<Output> {
@@ -55,7 +55,7 @@ extension Character: RegexComponent {
 }
 
 @available(SwiftStdlib 5.7, *)
-extension UnicodeScalar: RegexComponent {
+extension UnicodeScalar: @retroactive RegexComponent {
   public typealias Output = Substring
 
   public var regex: Regex<Output> {