[libtooling] add CXXThisExpr

Summary: Support CXXThisExpr. This came for free as it directly extends Expr.

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

Differential Revision: https://phabricator.fb.com/D1428792

Tasks: 3964417