[clang] [Clang] Add '-Warray-compare' flag for C++ below version 20 (PR #118031)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

This looks good to me and does what we discussed.  I'd prefer it if we could get some slightly improved diagnostic (just one that says quickly why it is problematic to compare arrays), and want someone else to take a quick peak (I've asked @shafik to do the final approval for me), else LGTM.

https://github.com/llvm/llvm-project/pull/118031