Add support for DenseMap/DenseSet count and find using const pointers

Summary:
Similar to SmallPtrSet, this makes find and count work with both const
referneces and const pointers.

Reviewers: dblaikie

Subscribers: llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D30713

llvm-svn: 297424