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GH-40607: [C++] Rename `Function::is_impure()` to `is_pure()` (#40608)

### Rationale for this change

Remove the property negation from the function name so that reasoning about the boolean value is easier.

"is_impure = false" vs "is_pure = true"
"is_impure" vs "!is_pure"

### What changes are included in this PR?

 - Renaming of the virtual function and overrides
 - Renaming of constructor parameters and class attributes
 - Swapping of the boolean constants
 - Replace `is_impure()` calls with `!is_pure()`
 - Undoing a documentation change done in #40396

### Are these changes tested?

By existing tests.

* GitHub Issue: #40607

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