clang-modernize: Introduce process-based parallelism

This patch introduces a new argument '-j' for transforming sources in
parallel. It defaults to 1. If there are >1 transforms to apply or >1
sources to transform clang-modernize will spawn sub-processes to
transform them.

FIXME: -perf and -summary don't work with this mode of operation as the
sub-processes don't communicate information back to the driver process
yet.

Added tests.