cpp11-migrate: Transforms collect timing data.

Using updated form of newFrontendActionFactory(), Transforms now automatically
measure, if requested, how long it takes to apply a MatchFinder to a source
file. Other per-transform overhead, e.g. applying replacements, is not
currently measured. This behaviour is disabled for now and soon will be
connected to a new command line arg.

llvm-svn: 182942