Resubmit "Refactor debuginfo-tests" again.

This was reverted due to the tests being run twice on some
build bots.  Each run had a slightly different configuration
due to the way in which it was being invoked.  This fixes
the problem (albeit in a somewhat hacky way).  Hopefully in
the future we can get rid of the workflow of running
debuginfo-tests as part of clang, and then this hack can
go away.

llvm-svn: 318697