Update the way llvm2cpp tests are done:

1. Make sure bytecode/assembly inputs are always redirected stdin so that
   the module name is <stdin>. This helps not get false negatives when the
   diff is done.
2. Scan the test file to determine if llvm-upgrade needs to be run.
3. Avoid running testings that are XFAIL'd because they'll cause a failure
   when run for llvm2cpp.
4. Get some better error message output.

llvm-svn: 35914