avoid Function instantiating std::decay over lambdas

Summary:
It turns out that `std::decay` becomes expensive when as widely instantiated as it is.

```name=trunk
$ foundation/scripts/run-perf-compile --syntax folly/futures/test/FutureTest.cpp
     6,336,421,499      instructions:uP
```

```name=branch
$ foundation/scripts/run-perf-compile --syntax folly/futures/test/FutureTest.cpp
     6,287,269,480      instructions:uP
```

Reviewed By: Gownta, luciang

Differential Revision: D32973204

fbshipit-source-id: 75d52fd5ced58f50518913d3c00709bcee27c87b

folly/Function.h

@@ -358,7 +358,7 @@ struct FunctionTraits<ReturnType(Args...)> {
   using NonConstSignature = ReturnType(Args...);
   using OtherSignature = ConstSignature;
 
-  template <typename F, typename R = CallableResult<std::decay_t<F>&, Args...>>
+  template <typename F, typename R = CallableResult<F&, Args...>>
   using IfSafeResult = IfSafeResultImpl<R, ReturnType>;
 
   template <typename Fun>
@@ -410,9 +410,7 @@ struct FunctionTraits<ReturnType(Args...) const> {
   using NonConstSignature = ReturnType(Args...);
   using OtherSignature = NonConstSignature;
 
-  template <
-      typename F,
-      typename R = CallableResult<const std::decay_t<F>&, Args...>>
+  template <typename F, typename R = CallableResult<const F&, Args...>>
   using IfSafeResult = IfSafeResultImpl<R, ReturnType>;
 
   template <typename Fun>
@@ -465,7 +463,7 @@ struct FunctionTraits<ReturnType(Args...) noexcept> {
   using NonConstSignature = ReturnType(Args...) noexcept;
   using OtherSignature = ConstSignature;
 
-  template <typename F, typename R = CallableResult<std::decay_t<F>&, Args...>>
+  template <typename F, typename R = CallableResult<F&, Args...>>
   using IfSafeResult = IfSafeResultImpl<R, ReturnType>;
 
   template <typename Fun>
@@ -517,9 +515,7 @@ struct FunctionTraits<ReturnType(Args...) const noexcept> {
   using NonConstSignature = ReturnType(Args...) noexcept;
   using OtherSignature = NonConstSignature;
 
-  template <
-      typename F,
-      typename R = CallableResult<const std::decay_t<F>&, Args...>>
+  template <typename F, typename R = CallableResult<const F&, Args...>>
   using IfSafeResult = IfSafeResultImpl<R, ReturnType>;
 
   template <typename Fun>