[Dominators] Introduce DomTreeNodeTraits to allow customization. (NFC)

This patch introduces DomTreeNodeTraits for customization. Clients can implement
DomTreeNodeTraitsCustom to provide custom ParentPtr, getEntryNode and getParent.
There's also a default specialization if DomTreeNodeTraitsCustom is not implemented,
that assume a Function-like NodeT. This is what is used for the existing DominatorTree
and MachineDominatorTree.

The main motivation for this patch is using DominatorTreeBase across all
regions of a VPlan, see D140513.

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D142162