[clang-tools-extra] [libc++] Implement ranges::iota (PR #68494)

[James E T Smith via cfe-commits]

jamesETsmith wrote:

@philnik777, I've added tests for `ranges::iota` to several of the `ranges_robust_against_*` tests where I thought it was appropriate. Below is a breakdown (mostly for my own tracking) of what I've done and what I'm still sorting through (`ranges_robust_against_proxy_iterators`). If you see anything that I missed or got wrong, just let me know.

- libcxx/test/std/algorithms
  - [X] libcxx/test/std/algorithms/ranges_result_alias_declarations.compile.pass.cpp
  - [X] libcxx/test/std/algorithms/ranges_robust_against_dangling.pass.cpp
  - [ ] libcxx/test/std/algorithms/ranges_robust_against_differing_projections.pass.cpp
    - I don't think we need this one, since we don't use projections
  - [ ] libcxx/test/std/algorithms/ranges_robust_against_nonbool_predicates.pass.cpp
    - I don't think we need this one since we don't use predicates
  - [ ] libcxx/test/std/algorithms/ranges_robust_against_omitting_invoke.pass.cpp
    - I don't think we need this one because we don't use invoke
  - [ ] libcxx/test/std/algorithms/ranges_robust_against_proxy_iterators.pass.cpp
    - This one is giving me trouble bc because 'Proxy<int &>' does not satisfy 'weakly_incrementable'
- libcxx/test/libcxx/
  - [ ] libcxx/test/libcxx/algorithms/ranges_robust_against_copying_comparators.pass.cpp
    - I don't think we need this since we don't use comparators
  - [ ] libcxx/test/libcxx/algorithms/ranges_robust_against_copying_projections.pass.cpp
    - I don't think we need this since we don't use projections


https://github.com/llvm/llvm-project/pull/68494