Downgrade let_unit_value to pedantic

Given that the false positive in rust-lang#1502 is marked E-hard and I don't have much hope of it getting fixed, I think it would be wise to disable this lint by default. I have had to suppress this lint in every substantial codebase (\>100k line) I have worked in. Any time this lint is being triggered, it's always the false positive case.

The motivation for this lint is documented as:

> A unit value cannot usefully be used anywhere. So binding one is kind of pointless.

with this example:

> ```rust
> let x = {
>     1;
> };
> ```

Sure, but the author would find this out via an unused_variable warning or from `x` not being the type that they need further down. If there ends up being a type error on `x`, clippy's advice isn't going to help get the code compiling because it can only run if the code already compiles.

changelog: Remove let_unit_value from default set of enabled lints

Don't lint `let_unit_value` when needed for type inferenece

fixes: rust-lang#1502

Pinging `@dtolnay.` I think this is enough to fix the issue. Do you have a good list crates to test this on?

changelog: Don't lint `let_unit_value` when needed for type inference

* Fix some typos.

Co-authored-by: pierwill <19642016+pierwill@users.noreply.github.com>

* Fix some typos.

Co-authored-by: pierwill <19642016+pierwill@users.noreply.github.com>