Rumors of test_debuginfo's death are slightly exaggerated.
Revert "Revert r113780, "Recognize .ll as input files. Handle %test_debuginfo on a RUN command line.""

This reverts commit r200145.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200258 91177308-0d34-0410-b5e6-96231b3b80d8