[OpaqueValues] Add frontend flag to disable.

To facilitate bringup and in preparation for enabling by default.

Author: nate-chandler

include/swift/Option/FrontendOptions.td

@@ -1166,6 +1166,9 @@ let Flags = [FrontendOption, NoDriverOption, HelpHidden, ModuleInterfaceOptionIg
 def enable_sil_opaque_values : Flag<["-"], "enable-sil-opaque-values">,
   HelpText<"Enable SIL Opaque Values">;
 
+def disable_sil_opaque_values : Flag<["-"], "disable-sil-opaque-values">,
+  HelpText<"Disable SIL Opaque Values">;
+
 def new_driver_path
   : Separate<["-"], "new-driver-path">, MetaVarName<"<path>">,
   HelpText<"Path of the new driver to be used">;

lib/Frontend/CompilerInvocation.cpp

@@ -2301,7 +2301,8 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
   Opts.EnableARCOptimizations &= !Args.hasArg(OPT_disable_arc_opts);
   Opts.EnableOSSAModules |= Args.hasArg(OPT_enable_ossa_modules);
   Opts.EnableOSSAOptimizations &= !Args.hasArg(OPT_disable_ossa_opts);
-  Opts.EnableSILOpaqueValues |= Args.hasArg(OPT_enable_sil_opaque_values);
+  Opts.EnableSILOpaqueValues = Args.hasFlag(
+      OPT_enable_sil_opaque_values, OPT_disable_sil_opaque_values, false);
   Opts.EnableSpeculativeDevirtualization |= Args.hasArg(OPT_enable_spec_devirt);
   Opts.EnableAsyncDemotion |= Args.hasArg(OPT_enable_async_demotion);
   Opts.EnableActorDataRaceChecks |= Args.hasFlag(