[ScopBuilder] Move Scop::init to ScopBuilder. NFC.

Scop::init is used only during SCoP construction. Therefore ScopBuilder
seems the more appropriate place for it. We integrate it onto its only
caller ScopBuilder::buildScop where some other construction steps
already took place.

Differential Revision: https://reviews.llvm.org/D32908

llvm-svn: 302276