introduce macro_arg intermediate query

Currently, when expanding macros, we look at the source code
directly (we invoke ast_id_to_node query via to_node method).

This is less then ideal, because it make us re-expand macros after
every source change.

This commit establishes a salsa-firewall: a query to get macro call's
token tree. Unlike the syntax tree, token tree changes only if we
actually modify the macro itself.

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