[llvm] [ADT] Adding bidirectional iterator functionality + unit tests (PR #160726)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

@FranciscoThiesen FYI, I edited the commit description to remove a mention because of how many notification this generates: https://discourse.llvm.org/t/forbidding-username-in-commits/86997

https://github.com/llvm/llvm-project/pull/160726