IRGen: Disable type layout based value witness generation at Onone

Type layout based witness generation can emit quite complex IR that will
lead to code bloat if the llvm optimizer is not run.

No need to use type layouts at Onone.

rdar://61155295

lib/Frontend/CompilerInvocation.cpp

@@ -1292,6 +1292,9 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
                                OPT_disable_type_layouts)) {
     Opts.UseTypeLayoutValueHandling
       = A->getOption().matches(OPT_enable_type_layouts);
+  } else if (Opts.OptMode == OptimizationMode::NoOptimization) {
+    // Disable type layouts at Onone except if explictly requested.
+    Opts.UseTypeLayoutValueHandling = false;
   }
 
   Opts.UseSwiftCall = Args.hasArg(OPT_enable_swiftcall);