adding clang sources and a setup script to compile clang

Summary: adding clang sources and a setup script to compile clang

Test Plan: run the script and then run make test and check that it works.

Reviewers: mathieubaudet

Reviewed By: mathieubaudet

Subscribers: cristianoc, mathieubaudet, irp

Differential Revision: https://phabricator.fb.com/D1522309

Tasks: 4864268