Refactor value clips code to use Usd_ClipSet

This change refactors helper functions and objects in
UsdStage to primarily deal with Usd_ClipSet objects
instead of individual clips. This lets us get rid of
some duplicated code and is a stepping stone for
moving these methods into Usd_ClipSet itself later.

This refactoring revealed places where we were checking
each clip in a clip set to determine if they applied to
a given layer stack site. Since the answer is the same
for all clips in a given clip set, we can just do this
check once per clip set instead.

(Internal change: 2076104)