Add flatc option to inhibit all warnings #6005 (#6301)

* Fix C/C++ Create<Type>Direct with sorted vectors

If a struct has a key the vector has to be sorted. To sort the vector
you can't use "const".

* Changes due to code review

* Improve code readability

* Add generate of JSON schema to string to lib

* option indent_step is supported

* Remove unused variables

* Fix break in test

* Fix style to be consistent with rest of the code

* Add option --no-warnings to inhibit all warnings

* Fix order of member initialization

* Add documentation for --no-warnings

docs/source/Compiler.md

@@ -229,5 +229,10 @@ Additional options:
 -   `--force-empty-vectors` : When serializing from object API representation, force
      vectors to empty rather than null.
 
+-   `--flexbuffers` : Used with "binary" and "json" options, it generates
+     data using schema-less FlexBuffers.
+
+-    `--no-warnings` : Inhibit all warning messages.
+
 NOTE: short-form options for generators are deprecated, use the long form
 whenever possible.