Place the "cfg checksum" around a test. This was recently added in April 2011 to

gcc, though I thought it was older (my gcc 4.4 has it as a local patch. Whoops!)
This fixes PR10589.

Also add some debugging statements.

Remove GcnoFiles, the mapping from CompilationUnit to raw_ostream. Now that we
start by iterating over each CU and descending into them, there's no need to
maintain a mapping.

llvm-svn: 145208