add comparison operators to must-use lint (under fn_must_use feature)

Although RFC 1940 is about annotating functions with `#[must_use]`, a
key part of the motivation was linting unused equality operators.

(See
https://github.com/rust-lang/rfcs/pull/1812#issuecomment-265695898—it
seems to have not been clear to discussants at the time that marking the
comparison methods as `must_use` would not give us the lints on
comparison operators, at least in (what the present author understood
as) the most straightforward implementation, as landed in #43728
(3645b062).)

To rectify the situation, we here lint unused comparison operators as
part of the unused-must-use lint (feature gated by the `fn_must_use`
feature flag, which now arguably becomes a slight (tolerable in the
opinion of the present author) misnomer).

This is in the matter of #43302.

src/test/ui/rfc_1940-must_use_on_functions/fn_must_use.rs

@@ -61,10 +61,9 @@ fn main() {
     m.need_to_use_this_method_value();
     m.is_even(); // trait method!
 
-    m.replace(3);
+    m.replace(3); // won't warn (annotation needs to be in trait definition)
 
-    2.eq(&3);
+    2.eq(&3); // comparison methods are `must_use`
 
-    // FIXME: operators should probably be `must_use` if underlying method is
-    2 == 3;
+    2 == 3; // lint includes comparison operators
 }

src/test/ui/rfc_1940-must_use_on_functions/fn_must_use.stderr

@@ -25,6 +25,12 @@ warning: unused return value of `EvenNature::is_even` which must be used: no sid
 warning: unused return value of `std::cmp::PartialEq::eq` which must be used
   --> $DIR/fn_must_use.rs:66:5
    |
-66 |     2.eq(&3);
+66 |     2.eq(&3); // comparison methods are `must_use`
    |     ^^^^^^^^^
 
+warning: unused comparison which must be used
+  --> $DIR/fn_must_use.rs:68:5
+   |
+68 |     2 == 3; // lint includes comparison operators
+   |     ^^^^^^
+