Add conversion from const folly::Optional to std::optional

Summary:
Previous version does not compile when converting from `const folly::Optional`
to `std::optional`. This happens in a typical `for (const auto& x : vec)` where
`x` is or contains `folly::Optional`.

Add a new `const` version in case people case about `Value&` ctor as opposed to
`const Value&`.

Reviewed By: iahs

Differential Revision: D26137442

fbshipit-source-id: 649be596b67a01c6d1f4be341aad65f1099b74a1

folly/Optional.h

@@ -188,7 +188,7 @@ class Optional {
     reset();
     return ret;
   }
-  explicit operator std::optional<Value>() & noexcept(
+  explicit operator std::optional<Value>() const& noexcept(
       std::is_nothrow_copy_constructible<Value>::value) {
     return storage_.hasValue ? std::optional<Value>(storage_.value)
                              : std::nullopt;

folly/test/OptionalTest.cpp

@@ -826,6 +826,10 @@ TEST(Optional, StdOptionalConversions) {
   EXPECT_EQ(*f, 42);
   EXPECT_TRUE(s);
 
+  const folly::Optional<int> fc = 12;
+  s = static_cast<std::optional<int>>(fc);
+  EXPECT_EQ(*s, 12);
+
   folly::Optional<std::unique_ptr<int>> fp = std::make_unique<int>(42);
   std::optional<std::unique_ptr<int>> sp(std::move(fp));
   EXPECT_EQ(**sp, 42);