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

[Tomas Matheson via cfe-commits]

================
@@ -11465,6 +11465,10 @@ static bool CheckMultiVersionFirstFunction(Sema &S, FunctionDecl *FD) {
   // otherwise it is treated as a normal function.
   if (TA && !TA->isDefaultVersion())
     return false;
+  // The target_version attribute only causes Multiversioning if this
+  // declaration is NOT the default version.
+  if (TVA && TVA->isDefaultVersion())
+    return false;
----------------
tmatheson-arm wrote:

```suggestion
  if (!TVA || TVA->isDefaultVersion())
    return false;
```

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