[libc] Expose APIGenerator.

Summary: This is split off from D79192 and exposes APIGenerator (renames to APIIndexer) for use in generating the integrations tests.

Reviewers: sivachandra

Reviewed By: sivachandra

Subscribers: tschuett, ecnelises, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D80832