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

[Aaron Ballman via cfe-commits]

================
@@ -692,6 +692,16 @@ 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_const_attr_with_pure_attr : Warning<
+  "'const' attribute imposes greater restrictions than 'pure', 'pure' attribute ignored">,
+  InGroup<IgnoredAttributes>;
+def warn_pure_attr_on_cxx_constructor : Warning<
+  "constructor cannot be '%select{pure|const}0', attribute ignored">,
----------------
AaronBallman wrote:

```suggestion
  "constructor cannot be '%select{pure|const}0'; attribute ignored">,
```

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