Rollup merge of #81645 - m-ou-se:panic-lint, r=estebank,flip1995

Add lint for `panic!(123)` which is not accepted in Rust 2021.

This extends the `panic_fmt` lint to warn for all cases where the first argument cannot be interpreted as a format string, as will happen in Rust 2021.

It suggests to add `"{}",` to format the message as a string. In the case of `std::panic!()`, it also suggests the recently stabilized
`std::panic::panic_any()` function as an alternative.

It renames the lint to `non_fmt_panic` to match the lint naming guidelines.

![image](https://user-images.githubusercontent.com/783247/106520928-675ea680-64d5-11eb-81f7-d8fa48b93a0b.png)

This is part of #80162.

r? ```@estebank```

compiler/rustc_ast/src/mut_visit.rs

@@ -28,7 +28,7 @@ pub trait ExpectOne<A: Array> {
 
 impl<A: Array> ExpectOne<A> for SmallVec<A> {
     fn expect_one(self, err: &'static str) -> A::Item {
-        assert!(self.len() == 1, err);
+        assert!(self.len() == 1, "{}", err);
         self.into_iter().next().unwrap()
     }
 }