Some projects need to add conditional dependencies on other projects.
compiler-rt is already doing this, and I attempted to add this to
debuginfo-tests when I ran into the ordering problem, that you can't
conditionally add a dependency unless that dependency's CMakeLists.txt
has already been run (which would allow you to say if (TARGET foo).

The solution to this seems to be to determine very early on the entire
set of projects which is enabled. This is complicated by the fact that
there are multiple ways to enable projects, and different tree layouts
(e.g. mono-repo, out of -tree, external, etc). This patch attempts to
centralize all of this into one place, and then updates compiler-rt to
demonstrate as a proof of concept how this can simplify code.

Differential Revision: https://reviews.llvm.org/D37637

llvm-svn: 313091