[Driver] Fix --repl argument.

The --repl option was incorrectly defined as "Separate" (option and
value separated by a space). This resulted in the option not being
picked up when no value was specified.

This patch fixes the driver so that `--repl` is recognized again. I
split the option into two:

 - A flag: `--repl` and `-r` which take no arguments.
 - A joined option: `--repl=<flags>` and `-r=<flags>` that forward its
   values to the repl.

This should match the driver's old behavior.

llvm-svn: 349371