Add test for non-concurrency async-main

This patch adds a quick test to verify that we emit a reasonable error
message when trying to use async-main without enabling concurrency or
parsing as stdlib instead of crashing.

Under both circumstances, the _Concurrency library won't be implicitly
imported.

Author: etcwilde

test/Concurrency/async_main_no_concurrency.swift

@@ -0,0 +1,8 @@
+// RUN: %target-typecheck-verify-swift -parse-as-library %s
+// RUN: %target-typecheck-verify-swift -parse-as-library -enable-experimental-concurrency -parse-stdlib %s
+
+@main struct Main {
+  // expected-error@+1:22{{'_Concurrency' module not imported, required for async main}}
+  static func main() async {
+  }
+}