fix(parser): fix variadic macro

[ObserverOfTime]

Problem

Compiling a parser with -pedantic warns that the REDUCE macro must have at least one variadic argument. This is innocuous since the macro does not expand into a function call, which prohibits trailing commas, but into a struct initialiser, where they are allowed. The warning can be silenced in clang but not in GCC. (MSVC doesn't care.)

Solution

Move all arguments into the variadic list.

Closes #3227

Alternatives

Separate into a macro with arguments and one without

That would make the generator more complicated.

Change the std to gnu11

No reason to do that just for this easily fixable warning.

[DavisVaughan]

I like this idea!