GH-49065: [C++] Remove unnecessary copies of shared_ptr in Type::BOOL and Type::NA at GrouperImpl

[HyukjinKwon]

Rationale for this change

The grouper code was creating a shared_ptr<DataType> for every key type, even when it wasn't needed. This resulted in unnecessary reference counting operations. For example, BooleanKeyEncoder and NullKeyEncoder don't require a shared_ptr in their constructors, yet we were creating one for every key of those types.

What changes are included in this PR?

Changed GrouperImpl::Make() to use TypeHolder references directly and only call GetSharedPtr() when needed by encoder constructors. This eliminates shared_ptr creation for Type::BOOL and Type::NA cases. Other encoder types (dictionary, fixed-width, binary) still require shared_ptr since their constructors take shared_ptr<DataType> parameters for ownership.

Are these changes tested?

Yes, existing tests.

Are there any user-facing changes?

No.

• GitHub Issue: [C++] Remove unnecessary copies of shared_ptr in Type::BOOL and Type::NA at GrouperImpl #49065

[github-actions]

⚠️ GitHub issue #49065 has been automatically assigned in GitHub to PR creator.

[kou]

+1