Add a flag to disable type layout based lowering

Author: aschwaighofer

include/swift/Option/FrontendOptions.td

@@ -677,4 +677,8 @@ def previous_module_installname_map_file
 
 def enable_type_layouts : Flag<["-"], "enable-type-layout">,
   HelpText<"Enable type layout based lowering">;
+
+def disable_type_layouts : Flag<["-"], "disable-type-layout">,
+  HelpText<"Disable type layout based lowering">;
+
 } // end let Flags = [FrontendOption, NoDriverOption, HelpHidden]

lib/Frontend/CompilerInvocation.cpp

@@ -1264,7 +1264,11 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
   Opts.EnableDynamicReplacementChaining |=
       Args.hasArg(OPT_enable_dynamic_replacement_chaining);
 
-  Opts.UseTypeLayoutValueHandling |= Args.hasArg(OPT_enable_type_layouts);
+  if (auto A = Args.getLastArg(OPT_enable_type_layouts,
+                               OPT_disable_type_layouts)) {
+    Opts.UseTypeLayoutValueHandling
+      = A->getOption().matches(OPT_enable_type_layouts);
+  }
 
   Opts.UseSwiftCall = Args.hasArg(OPT_enable_swiftcall);