Fix lookahead for effects specifiers in function types

Author: DougGregor

Sources/SwiftParser/Types.swift

@@ -815,7 +815,6 @@ extension Parser.Lookahead {
  if peek(isAtAnyIn: EffectSpecifier.self) != nil {
  var backtrack = self.lookahead()
  backtrack.consumeAnyToken()
- backtrack.consumeAnyToken()
  return backtrack.atFunctionTypeArrow()
  }
 

Tests/SwiftParserTest/DeclarationTests.swift

@@ -1048,6 +1048,15 @@ final class DeclarationTests: ParserTestCase {
  ],
  fixedSource: "func test() async throws(MyError){}"
  )
+
+ assertParse(
+ """
+ struct S<Element, Failure: Error> {
+ init(produce: @escaping () async throws(Failure) -> Element?) {
+ }
+ }
+ """
+ )
  }
 
  func testExtraneousRightBraceRecovery() {