Improve support for unusual C++ map types

Summary:
Some C++ maps have an unusual reference / const_reference type.  For example, std::flat_map, which uses:

std::pair<const key_type&, mapped_type&>
std::pair<const key_type&, const mapped_type&>

We would previously hit an error like this:

```
thrift/lib/cpp2/protocol/Cpp2Ops.h:43:3: error: static assertion failed due to requirement 'sizeof(const Foo&) == ~0ULL': (only Thrift-generated classes are serializable)
```

Here I strip off the cvref qualifiers.

Reviewed By: vitaut

Differential Revision: D59979468

fbshipit-source-id: dc3b60ceb485ba0b0688ba0ccf7ab0b6544aa61b

thrift/lib/cpp2/protocol/Cpp2Ops-inl.h

@@ -670,7 +670,7 @@ class Cpp2Ops<
     typedef typename std::remove_cv<
         typename std::remove_reference<decltype(*value->begin())>::type>::type
         PairType;
-    typedef typename PairType::second_type ValueType;
+    typedef folly::remove_cvref_t<typename PairType::second_type> ValueType;
     uint32_t xfer = 0;
     xfer += prot->writeMapBegin(
         Cpp2Ops<KeyType>::thriftType(),
@@ -692,7 +692,7 @@ class Cpp2Ops<
     typedef typename std::remove_cv<
         typename std::remove_reference<decltype(*value->begin())>::type>::type
         PairType;
-    typedef typename PairType::second_type ValueType;
+    typedef folly::remove_cvref_t<typename PairType::second_type> ValueType;
     value->clear();
     uint32_t size;
     protocol::TType keytype, valuetype;
@@ -712,7 +712,7 @@ class Cpp2Ops<
     typedef typename std::remove_cv<
         typename std::remove_reference<decltype(*value->begin())>::type>::type
         PairType;
-    typedef typename PairType::second_type ValueType;
+    typedef folly::remove_cvref_t<typename PairType::second_type> ValueType;
     uint32_t xfer = 0;
     xfer += prot->serializedSizeMapBegin(
         Cpp2Ops<KeyType>::thriftType(),
@@ -731,7 +731,7 @@ class Cpp2Ops<
     typedef typename std::remove_cv<
         typename std::remove_reference<decltype(*value->begin())>::type>::type
         PairType;
-    typedef typename PairType::second_type ValueType;
+    typedef folly::remove_cvref_t<typename PairType::second_type> ValueType;
     uint32_t xfer = 0;
     xfer += prot->serializedSizeMapBegin(
         Cpp2Ops<KeyType>::thriftType(),