[Migrated] Warn (or error) on a non-pub entrypoint

Issue automatically imported from old repo: https://github.com/EmbarkStudios/rust-gpu/issues/1140
Old labels: t: enhancement
Originally creatd by LegNeato on 2024-03-26T15:36:56Z

When setting #[spirv(fragment)] on a non-pub function, rust-gpu errors with an obtuse error:

error: error:0:0 - No OpEntryPoint instruction was found. This is only allowed if the Linkage capability is being used.

This case should be detected and warned or even made an error if it is not supported. Further, the error message should be fixed as suggested in https://github.com/EmbarkStudios/rust-gpu/issues/1139.

まず、非pub関数での #[spirv(fragment)] の処理を追跡し、issue 1139で説明されている関連エラーを確認します。このケースで、分かりにくいmissing-entry-pointメッセージではなく、明確な警告またはサポートされているエラーが生成されれば作業は完了です。