[clang] Add an extract-api driver option

This is the initial commit for the clang-extract-api RFC
<https://lists.llvm.org/pipermail/cfe-dev/2021-September/068768.html>
Add a new driver option `-extract-api` and associate it with a dummy
(for now) frontend action to set up the initial structure for
incremental works.

Differential Revision: https://reviews.llvm.org/D117809