[moveOnly] Add a frontend flag -enable-experimental-move-only to control usage of move only features.

These include _move and @_noImplicitCopy. I still need to wire up the parsing of
those behind this feature.

The reason that I am adding this now is that I am going to now need to make some
changes behind a feature flag and I have not yet needed to add one. The specific
reason I needed to add one here is to ensure that I properly guard inside _move
the call to Builtin.move so as to prevent a "cond_fail" incident.

P.S.: This work depends on experimental lexical lifetimes being enabled as well,
so I did that at the same time in this PR.

lib/Frontend/CompilerInvocation.cpp

@@ -443,6 +443,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableExperimentalDistributed |=
     Args.hasArg(OPT_enable_experimental_distributed);
 
+  Opts.EnableExperimentalMoveOnly |=
+    Args.hasArg(OPT_enable_experimental_move_only);
+
   Opts.EnableInferPublicSendable |=
     Args.hasFlag(OPT_enable_infer_public_concurrent_value,
                  OPT_disable_infer_public_concurrent_value,
@@ -1399,6 +1402,11 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
 
   Opts.EnableExperimentalLexicalLifetimes |=
       Args.hasArg(OPT_enable_experimental_lexical_lifetimes);
+  // If experimental move only is enabled, always enable lexical lifetime as
+  // well. Move only depends on lexical lifetimes.
+  Opts.EnableExperimentalLexicalLifetimes |=
+      Args.hasArg(OPT_enable_experimental_move_only);
+
   Opts.EnableCopyPropagation |= Args.hasArg(OPT_enable_copy_propagation);
   Opts.DisableCopyPropagation |= Args.hasArg(OPT_disable_copy_propagation);
   Opts.EnableARCOptimizations &= !Args.hasArg(OPT_disable_arc_opts);