[Sema] Make more overload candidate types use iterator_ranges (NFC)

I have a patch that adds another group of candidate types to
BuiltinCandidateTypeSet.  Currently two styles are in use: the older
begin/end pairs and the newer iterator_range approach.  I think the
group of candidates that I want to add should use iterator ranges,
but I'd also like to consolidate the handling of the new candidates
with some existing code that uses begin/end pairs.  This patch therefore
converts the begin/end pairs to iterator ranges as a first step.

No functional change intended.

Differential Revision: https://reviews.llvm.org/D92222