lld-link: Make /debugtype: option work better

- The code tried to pass false to split()'s KeepEmpty parameter, but
  instead passed it to MaxSplit. As a result, it would never split on
  commas. This has been broken since the flag was added in r278056.

- The code used getSpelling() for getting the argument's values, but
  getSpelling() always returns the `/debugtype:` prefix without any
  values. So if any /debugtype: flag was passed, it always resulted in
  an "unknown option:" warning. (The warning code then used the correct
  getValue() for printing the invalid option, so the warning looked
  kind of like it made sense.) This regressed in r342894.

Slightly improve the test coverage of this feature (but since I don't
know what this flag actually does, there's still no test for the correct
semantics), and add a comment to getSpelling() explaining what it does.

llvm-svn: 365182