Use different name for auto variable

Without this change GCC 5.4.0 failed to compile JSON.cpp with the error:

    .../llvm-project/llvm/lib/Support/JSON.cpp: In lambda function:
    .../llvm-project/llvm/lib/Support/JSON.cpp:291:29: error: use of 'V' before deduction of 'auto'
           for (const auto &V : *V.getAsArray())