dSYM generation should be off for all debug builds

[michaeleisel]

The use case provided for why one would have dSYM generation on is rare and should be opt-in. A lot of people leave dSYM generation on and don't realize it's adding 5+ seconds to every incremental build.

[fastred]

Agree. Would you like to create a PR with an improved explanation in that section?

[michaeleisel]

Eh, I'm too lazy