Misc fixes to expr_eagerness

[Jarcho]

Based on #17228

Changes:

• The handling of binary and unary operators have been merged to one arm each instead of relying on cascaded match guards. A little large, but it's much more clear how the overlapping cases are handled.
• Returning if there were any deref adjustments was just wrong. It would miss any lazy and unchangeable expressions.
• The visitor won't descend into const-evaluated expressions. This does mean something like cfg!(debug_assertions) || return can be suggested for eager evaluation which is notably abnormal code. I plan to rewrite this in a way that fixes this, but I need to finish const-evaluation changes first.
• References as operator arguments are now handled. They used to return Lazy, but they should return Eager. This is changing them to NoChange so we don't flip the suggestion over a single release.
• The two new const-evaluation functions don't make too much sense with the current API, but they match how I'm planning to change the API.

changelog: none

[rustbot]

Some changes occurred in clippy_lints/src/doc

cc @notriddle

[rustbot]

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