[clang] [clang][ExprConst] Add diagnostics for invalid binary arithmetic (PR #118475)

[Richard Smith via cfe-commits]

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@@ -91,11 +91,15 @@ def note_constexpr_pointer_subtraction_zero_size : Note<
   "subtraction of pointers to type %0 of zero size">;
 def note_constexpr_pointer_comparison_unspecified : Note<
   "comparison between '%0' and '%1' has unspecified value">;
+def note_constexpr_pointer_arith_unspecified : Note<
+  "arithmetic involving '%0' and '%1' has unspecified value">;
----------------
zygoloid wrote:

```suggestion
def note_constexpr_pointer_comparison_unspecified : Note<
  "comparison between pointers to unrelated objects '%0' and '%1' has unspecified value">;
def note_constexpr_pointer_arith_unspecified : Note<
  "arithmetic involving pointers to unrelated objects '%0' and '%1' has unspecified value">;
```

https://github.com/llvm/llvm-project/pull/118475