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GH-41114: [C++] Add is_validity_defined_by_bitmap() predicate (#41115)

### Rationale for this change

To make it easier to find bugs that are very likely to be silent in the codebas because users rarely use unions and REE types.

### What changes are included in this PR?

Adding the type predicate and two usages in `builder_nested.h`.

### Are these changes tested?

By the compilation process, since they are both `static_asserts`.
* GitHub Issue: #41114

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>