Don't use typeid without RTTI in UniqueInstance

Summary:
The class depends on RTTI. It's a sanity check that crashes if two instances of a singleton are created. So doing nothing in `-fno-rtti` code is fine.

Redo of D29630207 (https://github.com/facebook/folly/commit/160eb4d284eb67cc2641b6718c964dab8fc6486b)

Reviewed By: dtolnay

Differential Revision: D29684338

fbshipit-source-id: 38355df5297681329f227fd10570a816f4672b9b

folly/detail/UniqueInstance.h

@@ -20,6 +20,7 @@
 #include <typeinfo>
 
 #include <folly/CppAttributes.h>
+#include <folly/Portability.h>
 #include <folly/detail/StaticSingletonManager.h>
 
 namespace folly {
@@ -27,7 +28,7 @@ namespace detail {
 
 class UniqueInstance {
  public:
-#if __GNUC__ && __GNUC__ < 7 && !__clang__
+#if !FOLLY_HAS_RTTI || (__GNUC__ && __GNUC__ < 7 && !__clang__)
   explicit UniqueInstance(...) noexcept {}
 #else
   template <template <typename...> class Z, typename... Key, typename... Mapped>