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

[via cfe-commits]

wheatman wrote:

I found the test I would want to modify here (llvm-project/clang/test/Sema/warn-char-subscripts.c)
I will work on updating it

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