[clang] [clang] Add flag for relaxed pointer subtraction (PR #196392)

[via cfe-commits]

eleviant wrote:

> Thank you for this patch.
> 
> We need a release note and I would like to see an entry in the `clang/docs/UserManual.rst`, the `fstrict-bool` entry is a good example. Briefly outlining some useful scenarios as you did here might also be helpful: https://discourse.llvm.org/t/rfc-add-command-line-flag-to-make-pointer-subtraction-defined/90823/5?u=shafik

@shafik Thanks for taking a look. Added doc and release note.

https://github.com/llvm/llvm-project/pull/196392