Version of let_unit_value that is an early pass

[Manishearth]

For rust-lang/rust#44173

We should have a "use unit value" lint that checks for:

• () being assigned to a variable
• () being passed to a function

where the origin of the () is basically a semicolonless block

An example of an early pass is https://github.com/rust-lang-nursery/rust-clippy/blob/master/clippy_lints/src/precedence.rs

What we want is an early pass (runs on the AST before typechecking) with a check_expr and check_stmt that look for:

• assignment expressions
• let statements
• MethodCall expressions
• Call expressions

and then run a is_unit_expr check on their contents (the initializer for assignment/let, and each of the arguments for calls)

is_unit_expr will:

• if it's a Block, return true if the last statement is not an Expr statement
• if it's an if expr that has an else block, return true if both the if and else blocks have a non-expr final statement
• We can do something similar for match exprs if we wish.
• return false otherwise

We can also have it recurse, where if the last statement is a StmtKind::Expr, then recurse down and check if that expr evaluates to unit.

Then, if any of the is_unit_expr calls evaluates to true, throw the lint.

@zmanian is working on this