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

[Sanjoy Das]

What do you think about having a separate
LLVM_ENABLE_FAIL_FAST_ITERATORS macro?  That will be independent of
NDEBUG and will be explicitly documented to change ABI compatibility
of data structures.  This way LLDB can always (or never) build llvm
with LLVM_ENABLE_FAIL_FAST_ITERATORS and enable / disable the macro
when building LLDB sources consistently.  Doing this could be easier
than being consistent with NDEBUG.  This approach adds more complexity
on the LLVM side though, and I'd rather not do it if LLDB can be made
to be consistent on when it #defines NDEBUG.

-- Sanjoy