Only print registered targets for `--version`

Summary:
In D33900, I added printing of the registered targets in clang's
`PrintVersion` function, which is not only used for `--version` output,
but also for `-v` (verbose mode) and `-###`.  Especially the latter
seems to trip up some test cases, so it is probably better to only print
the registered targets for `--version`.

Reviewers: nemanjai, mehdi_amini

Reviewed By: nemanjai

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D33981

llvm-svn: 304899