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.

include/flatbuffers/base.h

@@ -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