Enable concurrency back-deployment in the compiler by default.

Remove the option that explicitly enables concurrency back-deployment,
and instead always enable its support in the compiler. Remove the use
of the extraneous CMake option as well.

lib/Frontend/CompilerInvocation.cpp

@@ -422,9 +422,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableExperimentalConcurrency |=
     Args.hasArg(OPT_enable_experimental_concurrency);
 
-  Opts.EnableExperimentalBackDeployConcurrency |=
-    Args.hasArg(OPT_enable_experimental_back_deploy_concurrency);
-
   Opts.EnableExperimentalNamedOpaqueTypes |=
       Args.hasArg(OPT_enable_experimental_named_opaque_types);