[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.

include/swift/Basic/LangOptions.h

@@ -320,6 +320,9 @@ namespace swift {
     /// Enable experimental 'distributed' actors and functions.
     bool EnableExperimentalDistributed = false;
 
+    /// Enable experimental 'move only' features.
+    bool EnableExperimentalMoveOnly = false;
+
     /// Disable the implicit import of the _Concurrency module.
     bool DisableImplicitConcurrencyModuleImport =
         !SWIFT_IMPLICIT_CONCURRENCY_IMPORT;

include/swift/Option/FrontendOptions.td

@@ -259,6 +259,10 @@ def enable_experimental_lexical_lifetimes :
   Flag<["-"], "enable-experimental-lexical-lifetimes">,
   HelpText<"Enable experimental lexical lifetimes">;
 
+def enable_experimental_move_only :
+  Flag<["-"], "enable-experimental-move-only">,
+  HelpText<"Enable experimental move only">;
+
 def enable_experimental_distributed :
   Flag<["-"], "enable-experimental-distributed">,
   HelpText<"Enable experimental 'distributed' actors and functions">;

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);

tools/sil-opt/SILOpt.cpp

@@ -108,6 +108,10 @@ static llvm::cl::opt<bool> EnableExperimentalLexicalLifetimes(
     "enable-experimental-lexical-lifetimes",
     llvm::cl::desc("Enable experimental lexical lifetimes."));
 
+static llvm::cl::opt<bool>
+EnableExperimentalMoveOnly("enable-experimental-move-only",
+                   llvm::cl::desc("Enable experimental distributed actors."));
+
 static llvm::cl::opt<bool>
 EnableExperimentalDistributed("enable-experimental-distributed",
                    llvm::cl::desc("Enable experimental distributed actors."));
@@ -424,6 +428,8 @@ int main(int argc, char **argv) {
     EnableExperimentalConcurrency;
   Invocation.getLangOptions().EnableExperimentalDistributed =
     EnableExperimentalDistributed;
+  Invocation.getLangOptions().EnableExperimentalMoveOnly =
+    EnableExperimentalMoveOnly;
 
   Invocation.getLangOptions().EnableObjCInterop =
     EnableObjCInterop ? true :
@@ -513,6 +519,11 @@ int main(int argc, char **argv) {
   SILOpts.DisableCopyPropagation = DisableCopyPropagation;
   SILOpts.EnableExperimentalLexicalLifetimes =
       EnableExperimentalLexicalLifetimes;
+  // Also enable lexical lifetimes if experimental move only is enabled. This is
+  // because move only depends on lexical lifetimes being enabled and it saved
+  // some typing ; ).
+  SILOpts.EnableExperimentalLexicalLifetimes |=
+    EnableExperimentalMoveOnly;
 
   serialization::ExtendedValidationInfo extendedInfo;
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> FileBufOrErr =