[CFG/BasicBlock] Rename succ_const to const_succ. [NFC]

Summary:
Rename `succ_const_iterator` to `const_succ_iterator` and
`succ_const_range` to `const_succ_range` for consistency with the
predecessor iterators, and the corresponding iterators in
MachineBasicBlock.

Reviewers: nicholas, dblaikie, nlewycky

Subscribers: hiraditya, bmahjour, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75952