[llvm] r231214 - Revert "[ADT] fail-fast iterators for DenseMap"

[Sanjoy Das]

> A separate define seems like the best we can do. The downside is that we
> can't turn this on by default in a standard +Asserts build, or we've just
> moved the ABI incompatibility problem to a different define.

This may be the most practical solution so far, but I dislike it
because it means a default +Asserts build won't get the kind of
verification it can and should.

-- Sanjoy