Fixed a FIXME; created a print method for Selectors that accepts a raw_ostream, and started using it in places it made sense.

No functional changes intended, just API cleanliness.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198428 91177308-0d34-0410-b5e6-96231b3b80d8