Properly add the Hashable conformance to ExperimentalFeatures

Author: DougGregor

CodeGeneration/Sources/generate-swift-syntax/templates/swiftparser/ExperimentalFeaturesFile.swift

@@ -20,7 +20,7 @@ let experimentalFeaturesFile = SourceFileSyntax(leadingTrivia: copyrightHeader)
     """
     extension Parser {
       @_spi(ExperimentalLanguageFeatures)
-      public struct ExperimentalFeatures: OptionSet, Sendable {
+      public struct ExperimentalFeatures: OptionSet, Hashable, Sendable {
         public let rawValue: UInt
         public init(rawValue: UInt) {
           self.rawValue = rawValue