[clang] [clang][analyzer] Add notes to PointerSubChecker (PR #95899)

[Donát Nagy via cfe-commits]

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@@ -154,12 +154,14 @@ void PointerSubChecker::checkPreStmt(const BinaryOperator *B,
     auto R =
         std::make_unique<PathSensitiveBugReport>(BT, Msg_MemRegionDifferent, N);
     R->addRange(B->getSourceRange());
-    if (DiffDeclL)
-      R->addNote("Array at the left-hand side of subtraction",
-                 {DiffDeclL, C.getSourceManager()});
-    if (DiffDeclR)
-      R->addNote("Array at the right-hand side of subtraction",
-                 {DiffDeclR, C.getSourceManager()});
+    if (DiffDeclL != DiffDeclR) {
----------------
NagyDonat wrote:

```suggestion
    if (DiffDeclL != DiffDeclR) {
      // The declarations may be identical even if the regions are different,
      // if they are field regions within different objects:
      //   struct { int array[10]; } a, b;
      //   do_something_with(a.array[5] - b.array[5]);
      // In this case the notes would be confusing, so don't emit them.
```
Add a comment that explains this `if (DiffDeclL != DiffDeclR)` check because otherwise it would be very surprising.

https://github.com/llvm/llvm-project/pull/95899