[SemaOverload] Use iterator_range to iterate over VectorTypes (NFC).

We can simplify the code a bit by using iterator_range instead of
plain iterators. Matrix type support here (added in 6f6e91d1)
already uses an iterator_range.

Reviewers: rjmccall, arphaman, jfb, Bigcheese

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D81138