[parser] Use Back2() where appropriate.

src/parsing/scanner.h

Index: src/parsing/scanner.h
diff --git a/src/parsing/scanner.h b/src/parsing/scanner.h
index ff5ca32b9bb1aa32f8d9886777caba12e245f6d5..b2b1a8a3f48a348b5fb8c44591d571749a0ac98f 100644
--- a/src/parsing/scanner.h
+++ b/src/parsing/scanner.h
@@ -617,6 +617,15 @@ class Scanner {
     c0_ = ch;
   }
 
+  // Same as PushBack(ch1); PushBack(ch2).
+  // - Potentially more efficient as it uses Back2() on the stream.
+  // - Uses char as parameters, since we're only calling it with ASCII chars in
+  //   practice. This way, we can avoid a few edge cases.
+  void PushBack2(char ch1, char ch2) {
+    source_->Back2();
+    c0_ = ch2;
+  }
+
   inline Token::Value Select(Token::Value tok) {
     Advance();
     return tok;