Fix the declaration printer to properly handle prototypes in C

Previously, we would take a declaration like void f(void) and print it
as void f(). That's correct in C++ as far as it goes, but is incorrect
in C because that converts the function from having a prototype to one
which does not.

This turns out to matter for some of our tests that use the pretty
printer where we'd like to get rid of the K&R prototypes from the test
but can't because the test is checking the pretty printed function
signature, as done with the ARCMT tests.