[clang] [Clang][Sema] Improve shuffle vector size mismatch diagnostic (PR #223768)

[Amr Hesham via cfe-commits]

================
@@ -6603,7 +6603,7 @@ ExprResult Sema::BuiltinShuffleVector(CallExpr *TheCall) {
 
       if (RHSVecType->getNumElements() != NumElements)
         return ExprError(Diag(TheCall->getBeginLoc(),
-                              diag::err_vec_builtin_incompatible_vector)
+                              diag::err_shufflevector_incompatible_vector)
----------------
AmrDeveloper wrote:

Yes, we can, I just made it consistent with the `__builtin_convertvector` diagnostic; the other option is to reuse `err_convertvector_incompatible_vector` and make the function name variable 🤔 

```
def err_convertvector_incompatible_vector : Error<
  "first two arguments to __builtin_convertvector must have the same number of elements">;

```

https://github.com/llvm/llvm-project/pull/223768