Auto merge of #93179 - Urgau:unreachable-2021, r=m-ou-se,oli-obk

Fix invalid special casing of the unreachable! macro

This pull-request fix an invalid special casing of the `unreachable!` macro in the same way the `panic!` macro was solved, by adding two new internal only macros `unreachable_2015` and `unreachable_2021` edition dependent and turn `unreachable!` into a built-in macro that do dispatching. This logic is stolen from the `panic!` macro.

~~This pull-request also adds an internal feature `format_args_capture_non_literal` that allows capturing arguments from formatted string that expanded from macros. The original RFC #2795 mentioned this as a future possibility. This feature is [required](https://github.com/rust-lang/rust/issues/92137#issuecomment-1018630522) because of concatenation that needs to be done inside the macro:~~
```rust
$crate::concat!("internal error: entered unreachable code: ", $fmt)
```

**In summary** the new behavior for the `unreachable!` macro with this pr is:

Edition 2021:
```rust
let x = 5;
unreachable!("x is {x}");
```
```
internal error: entered unreachable code: x is 5
```

Edition <= 2018:
```rust
let x = 5;
unreachable!("x is {x}");
```
```
internal error: entered unreachable code: x is {x}
```

Also note that the change in this PR are **insta-stable** and **breaking changes** but this a considered as being a [bug](https://github.com/rust-lang/rust/issues/92137#issuecomment-998441613).
If someone could start a perf run and then a crater run this would be appreciated.

Fixes https://github.com/rust-lang/rust/issues/92137

compiler/rustc_span/src/symbol.rs

@@ -1437,7 +1437,12 @@ symbols! {
         unmarked_api,
         unpin,
         unreachable,
+        unreachable_2015,
+        unreachable_2015_macro,
+        unreachable_2021,
+        unreachable_2021_macro,
         unreachable_code,
+        unreachable_display,
         unreachable_macro,
         unrestricted_attribute_tokens,
         unsafe_block_in_unsafe_fn,