[clang] [clang] Function type attribute to prevent CFI instrumentation (PR #135836)

[Erich Keane via cfe-commits]

================
@@ -12681,6 +12681,11 @@ def warn_noderef_on_non_pointer_or_array : Warning<
 def warn_noderef_to_dereferenceable_pointer : Warning<
   "casting to dereferenceable pointer removes 'noderef' attribute">, InGroup<NoDeref>;
 
+def warn_cast_discards_cfi_unchecked_callee
+    : Warning<"implicit conversion from %0 to %1 discards "
+              "`cfi_unchecked_callee` attribute">,
+      InGroup<CFIUncheckedCallee>;
----------------
erichkeane wrote:

```suggestion
      InGroup<DiagGroup<"cfi-unchecked-callee">>;
```
Then remove the group above.  Typically we don't add groups unless there are >1 uses of it.

https://github.com/llvm/llvm-project/pull/135836