libcdecl: Combine tag/typedef identifier rules.

libcdecl: Combine tag/typedef identifier rules.

There are actually no conflicts to add an empty production alongside
the rule for "struct", "union" and "enum", which allows the subsequent
reductions for "tag identifier" and "[nothing] identifier" to be done
with just one rule.

This replaces a relatively large action from the eliminated rule with
a very simple one.