[REQ] [Swift5] Improve Objective-C interoperability by using @objcMembers instead of @objc at class level

[tsanidas]

Describe the issue

When using the objcCompatible option, classes are generated with @objc at the class level. While using these classes from Objective-C, you will receive a compiler error when trying to access any property:
Property 'xyz' not found on object of type 'GeneratedSwiftClass *'

Describe the solution you'd like

I'd like to use @objcMembers instead of @objc in the mustache template file. I can send in a PR for this if we can get consensus on this change.

[wing328]

@tsanidas thanks for the suggestion.

cc @jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11)

[4brunu]

@tsanidas looks good to me 👍

[tsanidas]

Hi all. I've submitted a PR for this. Please see: #15188