[clang:ast] Avoid warning for unused var without assertions. (NFC) (#169822)

This PR avoids a compiler warning, which turns into an error with
`-Werror`, for a variable introduced in #169276 and only used in an
assertion (which is, thus, unused if compiled without assertions).

Signed-off-by: Ingo Müller <[email protected]>
Co-authored-by: Simon Pilgrim <[email protected]>

Author: ingomueller-net

clang/lib/AST/ExprConstant.cpp

@@ -12179,8 +12179,8 @@ static bool evalShiftWithCount(
   assert(Call->getNumArgs() == 2);
 
   QualType SourceTy = Call->getArg(0)->getType();
-  QualType CountTy = Call->getArg(1)->getType();
-  assert(SourceTy->isVectorType() && CountTy->isVectorType());
+  assert(SourceTy->isVectorType() &&
+         Call->getArg(1)->getType()->isVectorType());
 
   QualType DestEltTy = SourceTy->castAs<VectorType>()->getElementType();
   unsigned DestEltWidth = Source.getVectorElt(0).getInt().getBitWidth();