Rollup merge of #87599 - Smittyvb:concat_bytes, r=Mark-Simulacrum

Implement concat_bytes!

This implements the unstable `concat_bytes!` macro, which has tracking issue #87555. It can be used like:
```rust
#![feature(concat_bytes)]

fn main() {
    assert_eq!(concat_bytes!(), &[]);
    assert_eq!(concat_bytes!(b'A', b"BC", [68, b'E', 70]), b"ABCDEF");
}
```
If strings or characters are used where byte strings or byte characters are required, it suggests adding a `b` prefix. If a number is used outside of an array it suggests arrayifying it. If a boolean is used it suggests replacing it with the numeric value of that number. Doubly nested arrays of bytes are disallowed.

compiler/rustc_span/src/symbol.rs

@@ -439,6 +439,7 @@ symbols! {
         compiler_builtins,
         compiler_fence,
         concat,
+        concat_bytes,
         concat_idents,
         conservative_impl_trait,
         console,