Optimizer: extract the NamedReturnValueOptimization from CopyForwarding to a separate function pass

This allows to run the NamedReturnValueOptimization only late in the pipeline.
The optimization shouldn't be done before serialization, because it might prevent predictable memory optimizations in the caller after inlining.

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -758,6 +758,10 @@ static void addLowLevelPassPipeline(SILPassPipelinePlan &P) {
 
   addFunctionPasses(P, OptimizationLevelKind::LowLevel);
 
+  // The NamedReturnValueOptimization shouldn't be done before serialization.
+  // For details see the comment for `namedReturnValueOptimization`.
+  P.addNamedReturnValueOptimization();
+
   P.addDeadObjectElimination();
   P.addObjectOutliner();
   P.addDeadStoreElimination();