Introduce -enable-guaranteed-closure-contexts staging option.

lib/Frontend/CompilerInvocation.cpp

@@ -1002,6 +1002,8 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
   Opts.EmitProfileCoverageMapping |= Args.hasArg(OPT_profile_coverage_mapping);
   Opts.UseNativeSuperMethod |=
     Args.hasArg(OPT_use_native_super_method);
+  Opts.EnableGuaranteedClosureContexts |=
+    Args.hasArg(OPT_enable_guaranteed_closure_contexts);
 
   return false;
 }