Support SE-0317 async let

https://github.com/apple/swift-evolution/blob/main/proposals/0317-async-let.md

Author: taku0

swift-mode-lexer.el

@@ -437,6 +437,31 @@ Return nil otherwise."
                  "isolated")))
       nil)
 
+     ;; Before async
+     ;;
+     ;; Examples:
+     ;;
+     ;; func foo() async throws -> Void
+     ;; foo { () async throws -> void in }
+     ;; let f: () async throws -> Void = g
+     ;; get async throws {}
+     ;; async let x = foo()
+     ;;
+     ;; Suppresses implicit semicolon if and only if before let.
+     ;;
+     ;; Example:
+     ;;
+     ;; let a = f as (Int, Int)
+     ;;   async -> Int
+     ;; let b = t as (Int, Int)
+     ;; async
+     ;; let c = 1
+     ((equal (swift-mode:token:text next-token) "async")
+      (equal (swift-mode:token:text (save-excursion
+                                      (swift-mode:forward-token-simple)
+                                      (swift-mode:forward-token-simple)))
+             "let"))
+
      ;; Inserts semicolon before open curly bracket.
      ;;
      ;; Open curly bracket may continue the previous line, but we do not indent

test/swift-files/indent/declarations.swift

@@ -737,3 +737,13 @@ class Foo {
         return foo
     }
 }
+
+// async let
+
+func foo(f: @escaping (Int, Int) async -> Int, t: (Int, Int)) async {
+    let a = f as (Int, Int)
+      async -> Int
+    let b = t as (Int, Int)
+    async
+    let c = 1
+}