Fix lifetimes in StringReader.

Two different lifetimes are conflated. This doesn't matter right now,
but needs to be fixed for the next commit to work. And the more
descriptive lifetime names make the code easier to read.

compiler/rustc_parse/src/lexer/unicode_chars.rs

@@ -337,7 +337,7 @@ const ASCII_ARRAY: &[(&str, &str, Option<token::TokenKind>)] = &[
 ];
 
 pub(super) fn check_for_substitution(
-    reader: &StringReader<'_>,
+    reader: &StringReader<'_, '_>,
     pos: BytePos,
     ch: char,
     count: usize,