[libtooling] Support UsingDirectiveDecl

Summary: [libtooling] Support UsingDirectiveDecl

Test Plan:
DEBUG=1 make -C libtooling clean all
make -C libtooling clean test
make -C clang-ocaml clean
make -C clang-ocaml test

Reviewers: mathieubaudet

Reviewed By: mathieubaudet

Subscribers: cristianoc, martinoluca, ddino, dulmarod

Differential Revision: https://phabricator.fb.com/D1489227

Tasks: 3964417