[clang] Remove warnings from -Wchar-subscripts for known positive constants (PR #69061)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for this! The changes should come with a release note in `clang/docs/ReleaseNotes.rst` so users know about the change in behavior.

https://github.com/llvm/llvm-project/pull/69061