[DominatorTree] Simplify ChildrenGetter.

Summary:
Simplify ChildrenGetter to a simple wrapper around a GraphDiff call.
GraphDiff already handles nullptr in children, so the special casing in
clang can also be removed.

Reviewers: kuhar, dblaikie

Subscribers: llvm-commits, cfe-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D84713