LanguageServers/Cpp: Autocomplete declarations from included headers

We now also look at the available declarations from included header
files when autocompleting names.

Additionally, you can now request autocomplete on an empty token, which
brings up all available names, starting from the inner-most scope.

Userland/Libraries/LibCpp/Parser.cpp

@@ -664,7 +664,7 @@ bool Parser::done()
     return m_state.token_index == m_tokens.size();
 }
 
-StringView Parser::text_of_token(const Cpp::Token& token)
+StringView Parser::text_of_token(const Cpp::Token& token) const
 {
     ASSERT(token.m_start.line == token.m_end.line);
     ASSERT(token.m_start.column <= token.m_end.column);