Eagerly fetch typeck results when linting

[Jarcho]

The first commit avoids running the lint visitor if all lints were filtered out. In every other case the typeck results are accessed on every body making the delayed load pointless overhead.

[rustbot]

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

[rustbot]

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[Jarcho]

Turns out that was load bearing for rustdoc. That's now been worked around and documented properly.