Add PPFrame::isVolatile and PPFrame::setVolatile

Most wikitext is safe to parse once and then cache for when that same
wikitext is used again, such as for multiple transclusions of the same
template within a page. There are occasions, though, where some piece of
wikitext has side effects and so should not be cached; a prominent
example of such wikitext is the <ref> and <references> tags in Cite.php.

This change adds PPFrame::setVolatile so parser hooks such as <ref> and
<references> can indicate that they have done something that should not
be cached, and PPFrame::isVolatile so that callers of PPFrame::expand
can know when to avoid caching.

Bug: 46815
Bug: 31834
Change-Id: I95b3cf8781cf047cdb63da221cef45f3e7d1632e