Parse attribute parameters.
(parse-attribute-parameters parstate) → (mv erp attrparams span new-parstate)
This is only used if GCC extensions are supported.
See the ABNF grammar rule for
If parsing is successful, we return a list of zero or more expressions, which are the parameters. We re-use parse-argument-expressions to parse the zero or more comma-separated expressions. This parsing function does exactly what is needed here.