[clang] [clang][ptrauth] Warn about the use of a weak signing schema (PR #157779)

[Oliver Hunt via cfe-commits]

================
@@ -8357,6 +8357,16 @@ NamedDecl *Sema::ActOnVariableDeclarator(
                                    D.isFunctionDefinition());
   }
 
+  // Warn about the use of a weak pointer authentication schema on a variable
----------------
ojhunt wrote:

Yeah - I think that's an easier interface to use, otherwise all the code ends up having to do:

```cpp
if (ShouldDiagnoseBlah(...))
   DiagnoseBlah(...); // If you construct the diagnostic inline then as the checks become
                         // richer every call site has to work out the full diagnostic message
```

And yes, the result should be `void` - I was thinking in terms of Sema checks that can fail the diagnostics due to the error breaking further tests which is not the case here: warnings by definition don't cause this problem.

https://github.com/llvm/llvm-project/pull/157779