lld: make -v behave similarly to GNU ld.bfd

Previously, lld exited with an error status if the only option given to
the command was -v. GNU linkers gracefully exit in that case. This patch
makes lld behave like GNU.

Note that even with this patch, lld's -v and --version options behave
slightly differently than GNU linkers' counterparts. For example,
if you run ld.bfd -v -v, the version string is printed out twice.
But that is an edge case that I don't think we need to take care of.

Fixes https://bugs.llvm.org/show_bug.cgi?id=31582

Obtained from: LLVM r319717
MFC after: 1 week