Use standard llvm Debug.h support for debugging output.

Leave a quick "// Uncomment this." hint to enable the debug output in
tests. FIXME: figure out whether we want to enable debug command line
handling for all tests.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172608 91177308-0d34-0410-b5e6-96231b3b80d8