Re-revert "Refactor debuginfo-tests"

This is still broken because it causes certain tests to be
run twice with slightly different configurations, which is
wrong in some cases.

You can observe this by running:

  ninja -nv check-all | grep debuginfo-tests

And seeing that it passes clang/test and clang/test/debuginfo-tests
to lit, which causes it to run debuginfo-tests twice.  The fix is
going to involve either:

  a) figuring out that we're running in this "deprecated" configuration,
     and then deleting the clang/test/debuginfo-tests path, which should
     cause it to behave identically to before, or:
  b) make lit smart enough that it doesn't descend into a sub-suite if
     that sub-suite already has a lit.cfg file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@318486 91177308-0d34-0410-b5e6-96231b3b80d8