fdo69552 backward compatibility with optional arguments in calc functions

Hi Eike,

You reviewed https://gerrit.libreoffice.org/6767 and commented a.o.
"But more important, for backward compatibility file storage the parameter needs to be added when 
omitted so older versions still can calculate the function. See formula/source/core/api/token.cxx 
MissingConvention::isRewriteNeeded() and FormulaMissingContext::AddMoreArgs() and 
FormulaMissingContext::AddMissing()"

The problem I have here is that with CEILING() and FLOOR() the second argument (significance) is 
optional and has its default value (1 or -1) depending on the sign of the first argument (value). 
How do I obtain this value in formula/source/core/api/token.cxx 
FormulaMissingContext::AddMoreArgs() and FormulaMissingContext::AddMissing()?

The same problem may apply in sc/source/filter/excel/xeformula.cxx 
XclExpFmlaCompImpl::AppendTrailingParam(), but I take one hurdle at the time ;)

Any suggestions would be greatly appreciated.

Winfried