[driver] Refactor the driver so that a failing commands doesn't prevent
subsequent commands from being executed.

The diagnostics generation isn't designed for this use case, so add a note to
fix this in the very near future.  For now, just generated the diagnostics for
the first failing command.
Part of rdar://12984531

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173825 91177308-0d34-0410-b5e6-96231b3b80d8