message: Add support for absl::string_view when available
  For C++11 platforms, absl::string_view is sometimes available. This can
  be used for string_view when std::string_view is not available.
diff: @@ -212,6 +212,13 @@ namespace flatbuffers {
  typedef std::experimental::string_view string_view;
  }
  #define FLATBUFFERS_HAS_STRING_VIEW 1
  + // Check for absl::string_view
  + #elif __has_include("absl/strings/string_view.h")
  + #include "absl/strings/string_view.h"
  + namespace flatbuffers {
  + typedef absl::string_view string_view;
  + }
  + #define FLATBUFFERS_HAS_STRING_VIEW 1
  #endif
  #endif // __has_include
  #endif // !FLATBUFFERS_HAS_STRING_VIEW
  