Disable a couple of warnings for MSVC in folly::Function

Summary:
MSVC generates warnings if you define multiple assignment operators or multiple copy constructors on the same type. This is deliberate in `folly::Function`, so just disable the warnings for the struct instead.
Note that the warnings have to be disabled for the entire struct for them to actually be disabled. Disabling them just around the declarations of the constructors and operators in question does not actually disable the warnings.

Reviewed By: yfeldblum

Differential Revision: D3347746

fbshipit-source-id: abb53a1e62bcfb7ce02759a7ce8637d824a82081

folly/Function.h

@@ -224,6 +224,7 @@
 #include <utility>
 
 #include <folly/CppAttributes.h>
+#include <folly/Portability.h>
 
 namespace folly {
 
@@ -392,6 +393,9 @@ bool execBig(Op o, Data* src, Data* dst) {
 } // namespace function
 } // namespace detail
 
+FOLLY_PUSH_WARNING
+FOLLY_MSVC_DISABLE_WARNING(4521) // Multiple copy constructors
+FOLLY_MSVC_DISABLE_WARNING(4522) // Multiple assignment operators
 template <typename FunctionType>
 class Function final : private detail::function::FunctionTraits<FunctionType> {
   // These utility types are defined outside of the template to reduce
@@ -627,6 +631,7 @@ class Function final : private detail::function::FunctionTraits<FunctionType> {
     return Impl{std::make_shared<Function>(std::move(*this))};
   }
 };
+FOLLY_POP_WARNING
 
 template <typename FunctionType>
 void swap(Function<FunctionType>& lhs, Function<FunctionType>& rhs) noexcept {