[clang] [llvm] New calling convention preserve_none (PR #76868)

[Aaron Ballman via cfe-commits]

================
@@ -2868,6 +2868,11 @@ def M68kRTD: DeclOrTypeAttr {
   let Documentation = [M68kRTDDocs];
 }
 
+def PreserveNone : DeclOrTypeAttr {
+  let Spellings = [Clang<"preserve_none">];
----------------
AaronBallman wrote:

Calling conventions are somewhat odd in the attribute system, so I would do:
```
// let Subjects = [Function, ObjCMethod];
```
so there's some visual indication of what we meant but it doesn't have any effect. Many of the other calling conventions do this.

https://github.com/llvm/llvm-project/pull/76868