new lints around #[must_use] fns

`must_use_unit` lints unit-returning functions with a `#[must_use]`
attribute, suggesting to remove it.

`double_must_use` lints functions with a plain `#[must_use]`
attribute, but which return a type which is already `#[must_use]`,
so the attribute has no benefit.

`must_use_candidate` is a pedantic lint that lints functions and
methods that return some non-unit type that is not already
`#[must_use]` and suggests to add the annotation.

clippy_lints/src/returns.rs

@@ -318,6 +318,7 @@ fn attr_is_cfg(attr: &ast::Attribute) -> bool {
 }
 
 // get the def site
+#[must_use]
 fn get_def(span: Span) -> Option<Span> {
     if span.from_expansion() {
         Some(span.ctxt().outer_expn_data().def_site)