Introduce checking for ConcurrentValue conformance across actors.

When referring to an actor-isolated declaration from outside of the
actor, ensure that the types involved conform to the `ConcurrentValue`
protocol. Otherwise, produce a diagnostic stating that it is unsafe to
pass such types across actors.

Apply the same rule to local captures within concurrent code.

lib/Frontend/CompilerInvocation.cpp

@@ -385,6 +385,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.EnableExperimentalConcurrency |=
     Args.hasArg(OPT_enable_experimental_concurrency);
+  Opts.EnableExperimentalConcurrentValueChecking |=
+    Args.hasArg(OPT_enable_experimental_concurrent_value_checking);
 
   Opts.DisableImplicitConcurrencyModuleImport |=
     Args.hasArg(OPT_disable_implicit_concurrency_module_import);