[analyzer] Move RangeSet related declarations into the RangedConstraintManager header.

Summary: I could also move `RangedConstraintManager.h` under `include/` if you agree as it seems slightly out of place under `lib/`.

Patch by Réka Kovács

Reviewers: NoQ, george.karpenkov, dcoughlin, rnkovacs

Reviewed By: NoQ

Subscribers: mikhail.ramalho, whisperity, xazax.hun, baloghadamsoftware, szepet, a.sidorin, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D45920

llvm-svn: 333179