Auto merge of #112711 - Emilgardis:lit_byte_char, r=dtolnay

implement Literal::byte_character

without this, the only way to create a `LitKind::Byte` is by
doing `"b'a'".parse::<Literal>()`, this solves that by enabling
`Literal::byte_character(b'a')`

cc #71358

The tracking issue is #115268

Author: bors