[debuginfo-tests] Add support for moving debuginfo-tests from clang/test to llvm...

[debuginfo-tests] Add support for moving debuginfo-tests from clang/test to llvm/projects or monorepo.

Summary:
The new version of debuginfo-tests will have it's own
lit.cfg.py file which is incompatible with the one in clang/test.
This change supports both the old and new versions, and can be used
until the bots actually move debuginfo-tests to either clang/test or
the monorepo.

This is a prerequisite for D40971.

Reviewers: zturner, aprantl

Subscribers: mgorny, JDevlieghere, llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D41055

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@320494 91177308-0d34-0410-b5e6-96231b3b80d8