Support [x; n] expressions in concat_bytes!
Currently trying to use `concat_bytes!` with a repeating array value like `[42; 5]` results in an error:
```
error: expected a byte literal
--> src/main.rs:3:27
|
3 | let x = concat_bytes!([3; 4]);
| ^^^^^^
|
= note: only byte literals (like `b"foo"`, `b's'`, and `[3, 4, 5]`) can be passed to `concat_bytes!()`
```
This makes it so repeating array syntax can be used the same way normal arrays can be. The RFC doesn't explicitly mention repeat expressions, but it seems reasonable to allow them as well, since normal arrays are allowed.
It is possible to make the compiler get stuck compiling forever with `concat_bytes!([3; 999999999])`, but I don't think that's much of an issue since you can do that already with `const X: [u8; 999999999] = [3; 999999999];`.
Contributes to #87555.
03360be6b7