Skip `LintPass`es whose lints are all `Allow`

[kmcallister]

Since each LintPass declares the lints it can emit, we could speed up lint checking by skipping passes whose lints are all Allow. But some passes have side effects gathering information for other parts of the compiler, and those should always run.

[steveklabnik]

Triage: no change that I'm aware of.

[sanxiyn]

This doesn't seem workable, since LintPass only knows about lints's default levels, and lints's actual levels can be changed by attributes.

[steveklabnik]

Triage: no changes I'm aware of.

[Mark-Simulacrum]

I don't see how this can work since the level a lint warns can change with #[deny/warn/etc]. As such I'm going to close this.

[sanxiyn]

This can actually work, but workable strategy is now tracked at #42511.