[AST] Add a flag to guard exprimental multi-statement closures feature

include/swift/Basic/LangOptions.h

@@ -662,6 +662,10 @@ namespace swift {
     /// parameters of closures.
     bool EnableOneWayClosureParameters = false;
 
+    /// Enable experimental support for type inference through multi-statement
+    /// closures.
+    bool EnableMultiStatementClosureInference = false;
+
     /// See \ref FrontendOptions.PrintFullConvention
     bool PrintFullConvention = false;
   };

include/swift/Option/FrontendOptions.td

@@ -767,6 +767,10 @@ def experimental_one_way_closure_params :
   Flag<["-"], "experimental-one-way-closure-params">,
   HelpText<"Enable experimental support for one-way closure parameters">;
 
+def experimental_multi_statement_closures :
+  Flag<["-"], "experimental-multi-statement-closures">,
+  HelpText<"Enable experimental support for type inference in multi-statement closures">;
+
 def prebuilt_module_cache_path :
   Separate<["-"], "prebuilt-module-cache-path">,
   HelpText<"Directory of prebuilt modules for loading module interfaces">;

lib/Frontend/CompilerInvocation.cpp

@@ -954,6 +954,9 @@ static bool ParseTypeCheckerArgs(TypeCheckerOptions &Opts, ArgList &Args,
   Opts.EnableOneWayClosureParameters |=
       Args.hasArg(OPT_experimental_one_way_closure_params);
 
+  Opts.EnableMultiStatementClosureInference |=
+      Args.hasArg(OPT_experimental_multi_statement_closures);
+
   Opts.PrintFullConvention |=
       Args.hasArg(OPT_experimental_print_full_convention);