[clang] Fix -Wunused-variable in ASTContext.cpp (NFC)

/llvm-project/clang/lib/AST/ASTContext.cpp:12938:46: error: unused variable 'DY' [-Werror,-Wunused-variable]
    const auto *DX = cast<DecltypeType>(X), *DY = cast<DecltypeType>(Y);
                                             ^
1 error generated.