The OncePerASTToken machinery lets us automatically cancel "stale"
requests after a new one comes in. This avoid wasting time processing
requests that have been superceded, which is common for cursor-info, but
sometimes you really want to get results even later, so this commit adds
a way to opt out of the cancellation.
Incidentally, disable cancellation of name translation, which doesn't
really make sense and no one should be relying on that.
rdar://problem/31905379
