[libtooling] Support LambdaExpr

Summary: [libtooling] Support LambdaExpr

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: martinoluca, cristianoc, ddino, dulmarod

Differential Revision: https://phabricator.fb.com/D1474295

Tasks: 3964417