Rollup merge of #103919 - nnethercote:unescaping-cleanups, r=matklad

Unescaping cleanups

Some code improvements, and some error message improvements.

Best reviewed one commit at a time.

r? ````@matklad````

compiler/rustc_lexer/src/lib.rs

@@ -205,13 +205,13 @@ pub enum RawStrError {
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub enum Base {
     /// Literal starts with "0b".
-    Binary,
+    Binary = 2,
     /// Literal starts with "0o".
-    Octal,
-    /// Literal starts with "0x".
-    Hexadecimal,
+    Octal = 8,
     /// Literal doesn't contain a prefix.
-    Decimal,
+    Decimal = 10,
+    /// Literal starts with "0x".
+    Hexadecimal = 16,
 }
 
 /// `rustc` allows files to have a shebang, e.g. "#!/usr/bin/rustrun",