Parse zero or more argument expressions.
(parse-argument-expressions pstate) → (mv erp exprs span new-pstate)
This is called by parse-postfix-expression-rest, to parse the arguments of a function call. These are zero or more assignment expressions, as an optional non-empty sequence of assignment expressions in the grammar. That part of the grammar is left-recursive, which we handles as in other left-recursive parts of the grammar.
If the next token may start an expression, we parse an assignment expression, and then we call a separate function to parse any additional arguments. Otherwise, we return the empty list of argument expressions.