update according to the comments

Author: clingfei

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12951,7 +12951,7 @@ def err_builtin_invalid_arg_type: Error<
   "%plural{0:|: }3"
   "%plural{[0,3]:type|:types}1 (was %4)">;
 
-def err_bswapg_bitint_not_16bit_aligned : Error<
+def err_bswapg_invalid_bit_width : Error<
   "_BitInt type %0 (%1 bits) must be a multiple of 16 bits for byte swapping">;
 
 def err_builtin_trivially_relocate_invalid_arg_type: Error <

clang/lib/Sema/SemaChecking.cpp

@@ -2240,11 +2240,10 @@ static bool BuiltinBswapg(Sema &S, CallExpr *TheCall) {
         << ArgTy;
     return true;
   }
-  const auto *bitIntType = dyn_cast<BitIntType>(ArgTy);
-  if (bitIntType != nullptr) {
-    if (bitIntType->getNumBits() % 16 != 0 && bitIntType->getNumBits() != 8) {
-      S.Diag(Arg->getBeginLoc(), diag::err_bswapg_bitint_not_16bit_aligned)
-          << ArgTy << bitIntType->getNumBits();
+  if (const auto *BT = dyn_cast<BitIntType>(ArgTy)) {
+    if (BT->getNumBits() % 16 != 0 && BT->getNumBits() != 8) {
+      S.Diag(Arg->getBeginLoc(), diag::err_bswapg_invalid_bit_width)
+          << ArgTy << BT->getNumBits();
       return true;
     }
   }