[C++] Remove unnecessary copies of shared_ptr in Type::BOOL and Type::NA at GrouperImpl

[HyukjinKwon]

Describe the enhancement requested

arrow/cpp/src/arrow/compute/row/grouper.cc

Line 344 in 3e6182a

// TODO(wesm): eliminate this probably unneeded shared_ptr copy

Component(s)

C++

[kou]

Issue resolved by pull request 49066
#49066