[Concurrency] Put the flow-sensitive concurrent captures check behind a flag

We're not quite ready to commit to the flow-sensitive check that would
allow a concurrent function to read from a mutable local capture so
long as the captured variable wasn't changed after the point of
capture. Put it behind a flag and implement the more restrictive rule
(no access to mutable local captures in concurrent code). We can relax
it later.

lib/Frontend/CompilerInvocation.cpp

@@ -387,6 +387,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Args.hasArg(OPT_enable_experimental_concurrency);
   Opts.EnableExperimentalConcurrentValueChecking |=
     Args.hasArg(OPT_enable_experimental_concurrent_value_checking);
+  Opts.EnableExperimentalFlowSensitiveConcurrentCaptures |=
+    Args.hasArg(OPT_enable_experimental_flow_sensitive_concurrent_captures);
 
   Opts.DisableImplicitConcurrencyModuleImport |=
     Args.hasArg(OPT_disable_implicit_concurrency_module_import);