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

[Chuanqi Xu via cfe-commits]

================
@@ -692,6 +692,13 @@ def warn_maybe_falloff_nonvoid_function : Warning<
 def warn_falloff_nonvoid_function : Warning<
   "non-void function does not return a value">,
   InGroup<ReturnType>;
+def warn_pure_attr_on_cxx_constructor : Warning<
+  "constructor cannot be 'pure' (undefined behavior)">,
+  InGroup<IncorrectAttributeUsage>;
----------------
ChuanqiXu9 wrote:

```suggestion
  InGroup<<DiagGroup<"incorrect-pure-usage">>;
```

It looks a little bit far to invent `IncorrectAttributeUsage` group here

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