Cut non-const conversion overloads from Range

Summary: [Folly] Cut non-`const` conversion overloads from `Range`, for both `operator T` and `to<T>`: let them be `const` member functions so that the converted-to object cannot update `Range` internal state.

Reviewed By: terrelln, vitaut

Differential Revision: D8921428

fbshipit-source-id: eefc14b5fe2033cac43ebc274b27bdea95388488

folly/Range.h

@@ -474,14 +474,6 @@ class Range {
     return detail::value_before(e_);
   }
 
-  template <
-      typename Tgt,
-      std::enable_if_t<std::is_constructible<Tgt, Iter&, Iter&>::value, int> =
-          0>
-  FOLLY_CPP14_CONSTEXPR explicit operator Tgt() noexcept(
-      noexcept(Tgt(std::declval<Iter&>(), std::declval<Iter&>()))) {
-    return Tgt(b_, e_);
-  }
   template <
       typename Tgt,
       std::enable_if_t<
@@ -492,18 +484,6 @@ class Range {
     return Tgt(b_, e_);
   }
 
-  template <typename Tgt, typename... Args>
-  FOLLY_CPP14_CONSTEXPR auto to(Args&&... args) noexcept(noexcept(
-      Tgt(std::declval<Iter&>(),
-          std::declval<Iter&>(),
-          static_cast<Args&&>(args)...)))
-      -> decltype(
-          Tgt(std::declval<Iter&>(),
-              std::declval<Iter&>(),
-              static_cast<Args&&>(args)...)) {
-    return Tgt(b_, e_, static_cast<Args&&>(args)...);
-  }
-
   template <typename Tgt, typename... Args>
   constexpr auto to(Args&&... args) const noexcept(noexcept(
       Tgt(std::declval<Iter const&>(),