Workaround a release blocker after libc++ change (flutter/engine#43091)

The code breaks in C++20 mode after libc++ removes comparisons for `std::vector` and replaces them with 'operator <=>'.

See cl/542541552 for context.

engine/src/flutter/shell/platform/common/client_wrapper/include/flutter/encodable_value.h

@@ -215,6 +215,13 @@ class EncodableValue : public internal::EncodableValueVariant {
     }
     return std::get<int64_t>(*this);
   }
+
+  // Explicitly provide operator<, delegating to std::variant's operator<.
+  // There are issues with with the way the standard library-provided
+  // < and <=> comparisons interact with classes derived from variant.
+  friend bool operator<(const EncodableValue& lhs, const EncodableValue& rhs) {
+    return static_cast<const super&>(lhs) < static_cast<const super&>(rhs);
+  }
 };
 
 }  // namespace flutter