Comment parsing: Allow inline commands to have 0 or more than 1 argument

That's required to support `\n`, but can also be used for other commands.
We already had the infrastructure in place to parse a varying number of
arguments, we simply needed to generalize it so that it would work not
only for block commands.

This should fix #55319.

Reviewed By: gribozavr2

Differential Revision: https://reviews.llvm.org/D125429