fix: make `concat!` work with char
Fixes#12921
- I avoided making `unquote_str()` take char literals as well because it's depended on by another function `parse_string()` that's only supposed to take strings.
- Even with this patch, we don't output `\0` as `\u{0}` which #12921 pointed out ~~, but we're not actually responsible for serializing it but rowan is~~. They are functionally equivalent and I don't think it'd cause any confusion, but we *could* try escaping them before serialization (for reference, `rustc -Zunpretty=expanded`, which `cargo expand` uses under the hood, [makes use of `str::escape_default()`](3830ecaa8d/compiler/rustc_ast/src/util/literal.rs (L161)).
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