Revert "[ADT] fail-fast iterators for DenseMap"

This reverts commit 4b7263d855006988854036b4a4891fcf19aebe65.

r231125 http://reviews.llvm.org/D7931

This was causing many LLDB tests to fail on OS X, Linux, and FreeBSD.

https://bpaste.net/show/6a23e1f53623

llvm-svn: 231214