Add a lint to detect unconditional recursion.

Add a lint to detect unconditional recursion.

E.g. `fn foo() { foo() }`, or, more subtlely

    impl Foo for Box<Foo+'static> {
        fn bar(&self) {
            self.bar();
        }
    }

The compiler will warn and point out the points where recursion occurs,
if it determines that the function cannot return without calling itself.

Closes #17899.