[clang-ocaml/AST] Create functions get_{decl,stmt}_kind_string, add unit tests

Summary: [clang-ocaml/AST] Create functions get_{decl,stmt}_kind_string, add unit tests

Test Plan: make -C cclang-ocaml test

Reviewers: irp, ddino, dulmarod

Reviewed By: dulmarod

Subscribers: mathieubaudet

Differential Revision: https://phabricator.fb.com/D1524927