Allow opting-out of CustomDebugStringConvertible to reduce binary size

[bubski]

This observation comes from researching #1204.

It appears that CustomDebugStringConvertible's contribution to the binary size adds up and is not insignificant.
A quick check shows that removing it from Message, SimpleExtensionMap, AnyExtensionField would save us ~800kB.

For context, our project has around 1000 message definitions.
This was a modified version 1.17.0,
compiled with -Osize,
Xcode 13.2.1 13C100, Swift 5.5.2, Big Sur

Since conforming to CustomDebugStringConvertible does not seem essential, perhaps it could could be tied to a compilation flag?
I'd like to hear some initial feedback @thomasvl before getting into details.

[thomasvl]

This sounds sorta like another take on #1240.

With respect to compilation flags - those seems like they never work in practice with build systems - i.e. - how does one make that work (at scale) with SwiftPM/CocoaPods/etc.

TVL

[dflems]

@thomasvl: The DEBUG compilation condition seems fairly universal among build systems:

• CocoaPods adds DEBUG to swift compilation conditions under debug in the generated Xcode project (link)
• SwiftPM adds -DDEBUG to all compiles in the debug configuration (link)
• rules_swift adds it for dbg and fastbuild configs (link)

CustomDebugStringConvertible is meant for debugging so I think it would make sense to cut it out for release builds.

[thomasvl]

Yes, DEBUG would be less of an issue, I was taking the initial comment to be something developers directly controlled.

[dflems]

@thomasvl Took a crack at it here: #1246