Add support for absl::string_view when available (#5682)
Add support for absl::string_view when available (#5682)

For C++11 platforms, absl::string_view is sometimes available. This can
be used for string_view when std::string_view is not available.
DavadDi/flatbuffers,evolutional/flatbuffers,pjulien/flatbuffers,google/flatbuffers,pjulien/flatbuffers,evolutional/flatbuffers,google/flatbuffers,google/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,evolutional/flatbuffers,DavadDi/flatbuffers,google/flatbuffers,evolutional/flatbuffers,google/flatbuffers,google/flatbuffers,alexames/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,google/flatbuffers,pjulien/flatbuffers,DavadDi/flatbuffers,pjulien/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,DavadDi/flatbuffers,pjulien/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,pjulien/flatbuffers,google/flatbuffers,pjulien/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,pjulien/flatbuffers,google/flatbuffers,DavadDi/flatbuffers,evolutional/flatbuffers,pjulien/flatbuffers,evolutional/flatbuffers,google/flatbuffers,evolutional/flatbuffers,google/flatbuffers,DavadDi/flatbuffers,pjulien/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,pjulien/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,pjulien/flatbuffers,evolutional/flatbuffers