LibJS: Remove ParserState::lookahead_lexer

The lookahead lexer used by next_token() no longer needs to be kept
alive, since tokens created by Parser::next_token() now have any string
views guaranteed safe by the fact that they point into the one true
SourceCode provided by whoever set up the lexer.

Author: awesomekling

Libraries/LibJS/Parser.cpp

@@ -4291,10 +4291,8 @@ bool Parser::match_declaration(AllowUsingDeclaration allow_using) const
 
 Token Parser::next_token() const
 {
-    // We need to keep the lookahead lexer alive to prevent UAF on the lookahead token, as the token may hold a view
-    // into a short string stored on the stack.
-    m_state.lookahead_lexer = m_state.lexer;
-    return m_state.lookahead_lexer->next();
+    auto lookahead_lexer = m_state.lexer;
+    return lookahead_lexer.next();
 }
 
 bool Parser::try_match_let_declaration() const

Libraries/LibJS/Parser.h

@@ -288,7 +288,6 @@ class JS_API Parser {
 
     struct ParserState {
         Lexer lexer;
-        mutable Optional<Lexer> lookahead_lexer;
         [[nodiscard]] Token const& current_token() const { return lexer.current_token(); }
         bool previous_token_was_period { false };
         Vector<ParserError> errors;