[ADT] Enable reverse iteration for DenseMap

Reviewers: mehdi_amini, dexonsmith, dblaikie, davide, chandlerc, davidxl, echristo, efriedma

Reviewed By: dblaikie

Subscribers: rsmith, mgorny, emaste, llvm-commits

Differential Revision: https://reviews.llvm.org/D35043

llvm-svn: 311730