Avoid Unnecessary Builds

By default, stop the universe build if a key component fails.  This
avoids useless builds when we know a package is broken anyway.
Provide a --keep-going option to override this behavior.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173723 91177308-0d34-0410-b5e6-96231b3b80d8