[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">,
----------------
AaronBallman wrote:

```suggestion
  "'const' attribute imposes greater restrictions than 'pure'; 'pure' attribute ignored">,
```
Minor change, may have to re-flow the line to fit within 80 columns.

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