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

[via cfe-commits]

================
@@ -11889,6 +11889,13 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
     NewFD->setInvalidDecl();
   }
 
+  if (NewFD->hasAttr<PureAttr>() || NewFD->hasAttr<ConstAttr>()) {
+    if (isa<CXXConstructorDecl>(NewFD))
+      Diag(NewFD->getLocation(), diag::warn_pure_attr_on_cxx_constructor);
+    else if (NewFD->getReturnType()->isVoidType())
+      Diag(NewFD->getLocation(), diag::warn_pure_function_returns_void);
----------------
kelbon wrote:

I think it may be confusing, because name 'const' is similar to keyword 'const', specially for constructor
```cpp
struct A {
  A() const;
}
```

But i will check now to improve it

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