[clang-tidy] Add library for clang-tidy main function

Summary:
This library allows to create clang-tidy tools with custom checks outside of llvm repo
using prebuilt clang release tarball.

Test Plan:
Checked that clang-tidy works as before. New library exists in istall dir.

Reviewers: smeenai, gribozavr, stephanemoore

Subscribers: mgorny, xazax.hun, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D73300