[Sema] Diagnose tautological bounds checks (#120222)
This diagnoses comparisons like `ptr + unsigned_index < ptr` and `ptr +
unsigned_index >= ptr`, which are always false/true because addition of
a pointer and an unsigned index cannot wrap (or the behavior is
undefined).
This warning is intended to help find broken bounds checks (which must
be implemented in terms of uintptr_t instead).
Fixes https://github.com/llvm/llvm-project/issues/120214.
