Add `let_underscore_drop` lint.

This lint checks for statements similar to `let _ = foo`, where `foo` is
a type that implements `Drop`. These types of let statements cause the
expression in them to be dropped immediately, instead of at the end of
the scope. Such behavior can be surprizing, especially if you are
relying on the value to be dropped at the end of the scope. Instead, the
binding should be an underscore prefixed name (like `_unused`) or the
value should explicitly be passed to `std::mem::drop()` if the value
really should be dropped immediately.

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