[ADT] fail-fast iterators for DenseMap

Summary:
This patch is an attempt at making `DenseMapIterator`s "fail-fast".
Fail-fast iterators that have been invalidated due to insertion into
the host `DenseMap` deterministically trip an assert (in debug mode)
on access, instead of non-deterministically hitting memory corruption
issues.

Reviewers: dexonsmith, dberlin, ruiu, chandlerc

Reviewed By: chandlerc

Subscribers: yaron.keren, chandlerc, llvm-commits

Differential Revision: http://reviews.llvm.org/D7931

llvm-svn: 231035