Parse concurrency syntax when parsing for syntax-tree-only mode.

This allows the syntax parser library and SwiftSyntax to successfully
parse code using this experimental feature without requiring an API
to pass compiler flags into the parser.

test/Syntax/Parser/async.swift

@@ -0,0 +1,28 @@
+// Verify that async parses correctly via the parser lib even without the
+// experimental flag being set in LangOpts.
+//
+// REQUIRES: syntax_parser_lib
+// RUN: %swift-syntax-parser-test %s -dump-diags 2>&1 | %FileCheck %s
+
+func asyncGlobal1() async { }
+func asyncGlobal2() async throws { }
+
+typealias AsyncFunc1 = () async -> ()
+typealias AsyncFunc2 = () async throws -> ()
+
+func testTypeExprs() {
+  let _ = [() async -> ()]()
+  let _ = [() async throws -> ()]()
+}
+
+func testAwaitOperator() async {
+  let _ = __await asyncGlobal1()
+}
+
+func testAsyncClosure() {
+  let _ = { () async in 5 }
+  let _ = { () throws in 5 }
+  let _ = { () async throws in 5 }
+}
+
+// CHECK: 0 error(s) 0 warnings(s) 0 note(s)