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
    
    (cherry picked from commit c97f303880c26007b4e74e3fd0bde5ed802a25f1)