[cxx-interop] Emit -enable-experimental-cxx-interop into the serialized module interface

`.swiftinterface` files contain a list of compiler flags that were used to build the module. For example, the list includes `-enable-objc-interop`. It should also include `-enable-experimental-cxx-interop`, since it affects the way the module should be imported.

Author: egorzhdan

include/swift/Option/FrontendOptions.td

@@ -278,6 +278,10 @@ def enable_experimental_concurrency :
   Flag<["-"], "enable-experimental-concurrency">,
   HelpText<"Enable experimental concurrency model">;
 
+def enable_experimental_cxx_interop :
+  Flag<["-"], "enable-experimental-cxx-interop">,
+  HelpText<"Enable C++ interop code generation and config directives">;
+
 def enable_lexical_borrow_scopes :
   Joined<["-"], "enable-lexical-borrow-scopes=">,
   HelpText<"Whether to emit lexical borrow scopes (default: true)">,
@@ -825,11 +829,6 @@ def emit_sorted_sil : Flag<["-"], "emit-sorted-sil">,
 def emit_syntax : Flag<["-"], "emit-syntax">,
   HelpText<"Parse input file(s) and emit the Syntax tree(s) as JSON">, ModeOpt;
 
-def enable_experimental_cxx_interop :
-  Flag<["-"], "enable-experimental-cxx-interop">,
-  HelpText<"Enable C++ interop code generation and config directives">,
-  Flags<[FrontendOption, HelpHidden]>;
-
 def enable_cxx_interop :
   Flag<["-"], "enable-cxx-interop">,
   HelpText<"Alias for -enable-experimental-cxx-interop">,