add "experimental" to async demotion flag

Author: kavon

include/swift/Option/FrontendOptions.td

@@ -505,7 +505,7 @@ def disable_sil_partial_apply : Flag<["-"], "disable-sil-partial-apply">,
 def enable_spec_devirt : Flag<["-"], "enable-spec-devirt">,
   HelpText<"Enable speculative devirtualization pass.">;
 
-def enable_async_demotion : Flag<["-"], "enable-async-demotion">,
+def enable_async_demotion : Flag<["-"], "enable-experimental-async-demotion">,
   HelpText<"Enables an optimization pass to demote async functions.">;
 
 def disable_access_control : Flag<["-"], "disable-access-control">,

test/SILOptimizer/async-demotion/basic.swift

@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend -enable-async-demotion -disable-availability-checking -module-name main -O -emit-sil -primary-file %s | %FileCheck %s --implicit-check-not hop_to_executor
+// RUN: %target-swift-frontend -enable-experimental-async-demotion -disable-availability-checking -module-name main -O -emit-sil -primary-file %s | %FileCheck %s --implicit-check-not hop_to_executor
 
 // REQUIRES: swift_in_compiler
 // REQUIRES: concurrency