[clang] [BoundsSafety][Sema] Allow counted_by and counted_by_or_null on pointers where the pointee type is incomplete but potentially completable (PR #106321)

[Aaron Ballman via cfe-commits]

================
@@ -2068,13 +2068,55 @@ class Sema final : public SemaBase {
   bool CheckCountedByAttrOnField(FieldDecl *FD, Expr *E, bool CountInBytes,
                                  bool OrNull);
 
+  /// Perform Bounds Safety Semantic checks for assigning to a `__counted_by` or
+  /// `__counted_by_or_null` pointer type \param LHSTy.
+  ///
+  /// \param LHSTy The type being assigned to. Checks will only be performed if
+  ///              the type is a `counted_by` or `counted_by_or_null ` pointer.
+  /// \param RHSExpr The expression being assigned from.
+  /// \param Action The type assignment being performed
+  /// \param Loc The SourceLocation to use for error diagnostics
+  /// \param ComputeAssignee If provided this function will be called before
+  ///        emitting a diagnostic. The function should return the name of
+  ///        entity being assigned to or an empty string if this cannot be
+  ///        determined.
+  ///
+  /// \returns True iff no diagnostic where emitted, false otherwise.
+  bool BoundsSafetyCheckAssignmentToCountAttrPtr(
+      QualType LHSTy, Expr *RHSExpr, AssignmentAction Action,
+      SourceLocation Loc,
+      llvm::function_ref<std::string()> ComputeAssignee = nullptr);
+
+  /// Perform Bounds Safety Semantic checks for initializing a Bounds Safety
+  /// pointer.
+  ///
+  /// \param Entity The entity being initialized
+  /// \param Kind The kind of initialization being performed
+  /// \param Action The type assignment being performed
+  /// \param LHSTy The type being assigned to. Checks will only be performed if
+  ///              the type is a `counted_by` or `counted_by_or_null ` pointer.
+  /// \param RHSExpr The expression being used for initialization.
+  ///
+  /// \returns True iff no diagnostic where emitted, false otherwise.
+  bool BoundsSafetyCheckInitialization(const InitializedEntity &Entity,
+                                       const InitializationKind &Kind,
+                                       AssignmentAction Action,
+                                       QualType LHSType, Expr *RHSExpr);
+
+  /// Perform Bounds Safety semantic checks for uses of invalid uses counted_by
+  /// or counted_by_or_null pointers in \param E.
+  ///
+  /// \param E the expression to check
+  ///
+  /// \returns True iff no diagnostic where emitted, false otherwise.
+  bool BoundsSafetyCheckUseOfCountAttrPtr(Expr *E);
----------------
AaronBallman wrote:

```suggestion
  bool BoundsSafetyCheckUseOfCountAttrPtr(const Expr *E);
```
Can this be made `const`? I would expect a function named "check" doesn't need to mutate the expression.

https://github.com/llvm/llvm-project/pull/106321