[debuginfo-tests] Make debuginfo-tests work in a standard configuration.

Previously, debuginfo-tests was expected to be checked out into
clang/test and then the tests would automatically run as part of
check-clang.  This is not a standard workflow for handling
external projects, and it brings with it some serious drawbacks
such as the inability to depend on things other than clang, which
we will need going forward.

The goal of this patch is to migrate towards a more standard
workflow.  To ease the transition for build bot maintainers,
this patch tries not to break the existing workflow, but instead
simply deprecate it to give maintainers a chance to update
the build infrastructure.

Differential Revision: https://reviews.llvm.org/D39605

llvm-svn: 317925