[ScopBuilder/ScopInfo] Move ScopStmt::buildAccessRelations to ScopBuilder. NFC.

This method is only called in the SCoP building phase.
Therefore it fits better into ScopBuilder to separate
SCoP-construction from SCoP modeling.

This mostly mechanical change makes ScopBuilder directly access some of
ScopStmt/MemoryAccess private fields. We add ScopBuilder as a friend
class and will add proper accessor functions sometime later.

llvm-svn: 312115