test-release.sh: Update to fetch source from GitHub

Summary:
This also changes the test-release.sh script to build using the monorepo
layout instead of copying sub-projects into llvm/tools or llvm/projects.

Reviewers: jdoerfert, hans

Reviewed By: hans

Subscribers: hans, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70353