Nonlinearity in `check_mod_privacy` pass for a large number of impls

[weiznich]

This is a followup issue to rust-lang/rustc-perf#807, where I've suggested to add diesel with the 128-column-tables enabled to the rustc-perf test-suite as this is taking quite a lot of time to compile. As part of that request we discovered that the check_mod_privacy pass scales non-linear with the number of code lines. See the linked PR for the exact numbers. I suspect that this is somehow related to a large number of trait impls for tuples of various sizes.

@rustbot modify labels: +I-slow