[clang] [clang][FMV] Do not omit explicit default target_version attribute. (PR #96628)

[Tomas Matheson via cfe-commits]

================
@@ -11576,22 +11584,6 @@ static bool CheckTargetCausesMultiVersioning(Sema &S, FunctionDecl *OldFD,
     }
   }
 
-  if (NewTVA) {
----------------
tmatheson-arm wrote:

If you are reasoning based on that, it might be good to add an assert checking it:
```
assert(!OldTVA || OldTVA->isDefaultVersion());
```

https://github.com/llvm/llvm-project/pull/96628