[tests] Add Serialization tests for async let any_patter bit encode/decode

Author: LucianoPAlmeida

test/Serialization/Inputs/def_async.swift

@@ -1 +1,11 @@
 public func doSomethingBig() async -> Int { return 0 }
+
+@_alwaysEmitIntoClient
+public func doSerializedAsyncLet() async {
+  async let _ = ()
+}
+
+@_alwaysEmitIntoClient
+public func doSerializedAsyncLetTyped() async {
+  async let _ : () = ()
+}

test/Serialization/async.swift

@@ -11,3 +11,13 @@ import def_async
 func testDoSomethingBig() {
   let _: () -> Int = doSomethingBig // expected-error{{invalid conversion from 'async' function of type '() async -> Int' to synchronous function type '() -> Int'}}
 }
+
+// Verify serialization of async let _ = <expression> pattern
+func testSerializedAsyncLet() async {
+  await doSerializedAsyncLet()
+} 
+
+// Verify serialization of async let _ : <type> = <expression> pattern
+func testSerializedAsyncLetTyped() async {
+  await doSerializedAsyncLetTyped()
+}