Add a new driver mode to dump compiler feature and extension options.

Add the ability to dump compiler option-related information to a JSON file via the -compiler-options-dump option. Specifically, it dumps the features/extensions lists -- however, this output could be extended to other information should it be useful. In order to support features and extensions, I moved them into a .def file so that we could build the various lists we care about from them without a significant increase in maintenance burden.

llvm-svn: 333653