Add experimental-async-top-level flag

Protect this feature behind a feature flag until it's ready for
prime-time.

Author: etcwilde

include/swift/Basic/LangOptions.h

@@ -390,6 +390,9 @@ namespace swift {
     /// cases.
     bool EnableNonFrozenEnumExhaustivityDiagnostics = false;
 
+    /// Enable making top-level code support concurrency
+    bool EnableExperimentalAsyncTopLevel = false;
+
     /// Regex for the passes that should report passed and missed optimizations.
     ///
     /// These are shared_ptrs so that this class remains copyable.

include/swift/Option/FrontendOptions.td

@@ -297,6 +297,10 @@ def enable_experimental_eager_clang_module_diagnostics :
 
 def enable_resilience : Flag<["-"], "enable-resilience">,
      HelpText<"Deprecated, use -enable-library-evolution instead">;
+
+def enable_experimental_async_top_level :
+  Flag<["-"], "enable-experimental-async-top-level">,
+  HelpText<"Enable experimental concurrency in top-level code">;
 }
 
 // HIDDEN FLAGS

lib/Frontend/CompilerInvocation.cpp

@@ -470,11 +470,16 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.DisableImplicitConcurrencyModuleImport |=
     Args.hasArg(OPT_disable_implicit_concurrency_module_import);
 
+  Opts.EnableExperimentalAsyncTopLevel |=
+    Args.hasArg(OPT_enable_experimental_async_top_level);
+
   /// experimental distributed also implicitly enables experimental concurrency
   Opts.EnableExperimentalDistributed |=
     Args.hasArg(OPT_enable_experimental_distributed);
   Opts.EnableExperimentalConcurrency |=
     Args.hasArg(OPT_enable_experimental_distributed);
+  Opts.EnableExperimentalConcurrency |=
+    Args.hasArg(OPT_enable_experimental_async_top_level);
 
   Opts.DiagnoseInvalidEphemeralnessAsError |=
       Args.hasArg(OPT_enable_invalid_ephemeralness_as_error);