init: Simplify struct BuiltinArguments

Make the code that creates BuiltinArguments instances easier to read by
using initializer lists instead of constructor calls. Remove the
BuiltinArguments constructors.

Change-Id: I6cf215a81d298cf7e524e22fb75db820e0225c9a
Signed-off-by: Bart Van Assche <bvanassche@google.com>