[clang] [CIR][ABI] Add restrict, nonnull, and ReturnsNonNull attributes (PR #188281)

[Andy Kaylor via cfe-commits]

================
@@ -398,8 +398,13 @@ void CIRGenModule::constructAttributeList(
     attrs.set(cir::CIRDialect::getSideEffectAttrName(),
               cir::SideEffectAttr::get(&getMLIRContext(), sideEffect));
 
-    // TODO(cir): When doing 'return attrs' we need to cover the Restrict and
-    // ReturnsNonNull attributes here.
+    if (targetDecl->hasAttr<ReturnsNonNullAttr>())
----------------
andykaylor wrote:

```suggestion
    if (targetDecl->hasAttr<ReturnsNonNullAttr>() && !codeGenOpts.NullPointerIsValid)
```

https://github.com/llvm/llvm-project/pull/188281