[libc++] [test] Test that unordered_*::swap/move/assign does not invalidate iterators.

And remove the dedicated debug-iterator tests; we want to test this in all modes.
We have a CI step for testing the whole test suite with `--debug_level=1` now.

Part of https://reviews.llvm.org/D102003

GitOrigin-RevId: f42355e17c3f3d1d099d028a388796a64724ffdb
16 files changed