[C++] "nonexistent" or "non-existent"

[Sutou Kouhei]

Hi,

I found that we use both "nonexistent" and "non-existent" in
our C++ code base. I think that we should use one of them
instead of mixing them.

"nonexistent":

  * Public API:
    * cpp/src/plasma/: plasma::PlasmaErrorCode::PlasmaObjectNonexistent

"non-existent":

  * Public API:
    * cpp/src/arrow/filesystem/: arrow::fs::FileType::NonExistent
  * Internal:
    * cpp/src/arrow/util/io_util.h: allow_non_existent
  * Test:
    * cpp/src/gandiva/tests/: non_existent_function


Which should we use?
(Personally, I prefer "nonexistent" to "non-existent".)


Thanks,
--
kou