Change interface for llvm-projs and llvm-sync

llvm-projs and llvm-sync used to have very inconsistent interfaces. This commit
updates the interfaces so that llvm-sync only synchronizes the repos that are
linked into the tree by llvm-projs (this makes it consistent with llvm-branch
and other scripts) and also revamps the interface provided by llvm-projs to
allow some long options for projects that are frequently used together as well
as short options for fine tuning whether or not to link each project.

Change-Id: Iffdbc8d7956473d09c8161d6fb773c530e5b4d79
2 files changed