[clang] [Clang][Sema] Diagnose invalid 'pure'/'const' attributes on writes (PR #217817)

[Oliver Hunt via cfe-commits]

================
@@ -10,16 +10,14 @@ __attribute__((pure)) int func2(void);
 
 #ifdef __cplusplus
 struct CppTest {
-  // They are fine on member functions.
-  __attribute__((const)) int func();
+// They are fine on member functions.
+__attribute__((const)) int func();
   [[gnu::pure]] int other_func();
-  
-  int another();
-  
-  // Constructors and destructors are not allowed though because they
-  // notionally return void.
+int another();
+// Constructors and destructors are not allowed though because they
+// notionally return void.
   [[__gnu__::__const__]] CppTest(); // expected-warning {{'const' attribute on function returning 'void'; attribute ignored}}
-  __attribute__((pure)) ~CppTest(); // expected-warning {{'pure' attribute on function returning 'void'; attribute ignored}}
+__attribute__((pure)) ~CppTest(); // expected-warning {{'pure' attribute on function returning 'void'; attribute ignored}}
----------------
ojhunt wrote:

unneeded change

https://github.com/llvm/llvm-project/pull/217817