[clang] Warning for incorrect useof 'pure' attribute (PR #78200)

[Erich Keane via cfe-commits]

================
@@ -11792,6 +11792,26 @@ static bool CheckMultiVersionFunction(Sema &S, FunctionDecl *NewFD,
                                          OldDecl, Previous);
 }
 
+static void CheckFunctionDeclarationAttributesUsage(Sema &S,
+                                                    FunctionDecl *NewFD) {
+  bool IsPure = NewFD->hasAttr<PureAttr>();
+  bool IsConst = NewFD->hasAttr<ConstAttr>();
+
+  if (IsPure && IsConst) {
+    S.Diag(NewFD->getLocation(), diag::warn_const_attr_with_pure_attr);
+    NewFD->dropAttr<PureAttr>();
+  }
+  if (IsPure || IsConst) {
+    // constructors and destructors also functions which returns void
----------------
erichkeane wrote:

```suggestion
    // Constructors and destructors are functions which return void, so are handled here as well.
```

https://github.com/llvm/llvm-project/pull/78200