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GH-45301: [C++] Change PrimitiveArray ctor to protected (#45444)



### Rationale for this change

This patch handles the case in GH-45301, changing the ctor for PrimitiveArray to private.

### What changes are included in this PR?

change the ctor for PrimitiveArray to private.

### Are these changes tested?

Yes

### Are there any user-facing changes?

This PR makes protected a constructor that was public. Calling this constructor outside of subclasses resulted in incorrect code.

* GitHub Issue: #45301

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: mwish <maplewish117@gmail.com>