[clang-ocaml] Use "classic" sum types instead of Ocaml variants for the four main AST types

Summary:
[clang-ocaml] Use "classic" sum types instead of Ocaml variants for the four main AST types

This is kinda of cosmetic and will impact the code from Infer so tell me what you think.

Migration plan:
* add `open Clang_ast_t`
* remove the backquote the beginning of every `.*Decl`, `.*Stmt`, `.*Expr`, `.*Comment`
* for every attribute, remove the backquote at the beginning and add `Attr` at the end. (Hopefully Infer does not use attributes yet.)

Test Plan: make -C libtooling test && make -C clang-ocaml test

Reviewers: dulmarod, ddino, irp

Reviewed By: irp

Subscribers: mathieubaudet, pad

Differential Revision: https://phabricator.fb.com/D1470223
[libtooling] ASTExporter: remove dumpName()

Summary:
[libtooling] ASTExporter: remove dumpName()

First diff of a serie to remove ADT "inherit" statements and simplify the code.

Test Plan: make -C libtooling test && make -C clang-ocaml test

Reviewers: irp

Reviewed By: irp

Differential Revision: https://phabricator.fb.com/D1471199
[libtooling] ASTExporter: make source_range non-optional, do not "inherit"

Summary: [libtooling] ASTExporter: make source_range non-optional, do not "inherit"

Test Plan: make -C libtooling test && make -C clang-ocaml test

Reviewers: irp

Reviewed By: irp

Differential Revision: https://phabricator.fb.com/D1471201
[libtooling] ASTExporter: remove "inherit _full_comment"

Summary: [libtooling] ASTExporter: remove "inherit _full_comment"

Test Plan: make -C libtooling test && make -C clang-ocaml test

Reviewers: irp

Reviewed By: irp

Differential Revision: https://phabricator.fb.com/D1471202