[clang] [BoundsSafety][NFC] Add counted_by type-shape validation helper (PR #227774)

[Dan Liew via cfe-commits]

================
@@ -710,6 +710,12 @@ class LangOptions : public LangOptionsBase {
     return ConvergentFunctions;
   }
 
+  /// Return true if bounds attributes follow the -fbounds-safety programming
+  /// model. Downstream, which has the full -fbounds-safety implementation, uses
+  /// this predicate to gate behavior that differs from upstream. Remove it once
+  /// that divergence is resolved.
+  bool hasBoundsSafetyAttributes() const { return BoundsSafety; }
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delcypher wrote:

We may want to consider renaming `hasBoundsSafetyAttributes()` (maybe a separate PR) because the name is confusing because `__counted_by` can be used without any kind of `-fbounds-safety` mode.

For now we might want to consider rewording to comments (which I also think are confusing) and also refer to a GitHub issue that tracks upstreaming "-fbounds-safety attribute-only mode".

Suggested rewording:

```
/// Return true if using -fbounds-safety in full or attribute-only mode
bool hasBoundsSafetyAttributes() const { 
  // TODO: Upstream attribute-only mode (refer to github issue here)
  return BoundsSafety;
 }
```

https://github.com/llvm/llvm-project/pull/227774