[Dominators] Add PDT constructor from Function

Summary: This patch adds a PDT constructor from Function and lets codes previously using a local class to do this use PostDominatorTree class directly.

Reviewers: davide, kuhar, grosser, dberlin

Reviewed By: kuhar

Author: NutshellySima

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46709

llvm-svn: 333102