[clang] [ConstEval] Fix crash when comparing strings past the end (PR #137078)

[Richard Smith via cfe-commits]

https://github.com/zygoloid approved this pull request.

Thanks for the fix!

I was wondering if the new code paths should actually return `true` not `false`, but I think it makes sense that for a comparison like `"hello" + 6 == "world"`, we say that the strings are *not* potentially-overlapping but we instead detect the case of comparing a past-the-end pointer to one complete object against the start of another complete object.

https://github.com/llvm/llvm-project/pull/137078