qapi-visit: Split off visit_type_FOO_fields forward decl

qapi-visit: Split off visit_type_FOO_fields forward decl

We generate a static visit_type_FOO_fields() for every type
FOO.  However, sometimes we need a forward declaration. Split
the code to generate the forward declaration out of
gen_visit_implicit_struct() into a new gen_visit_fields_decl(),
and also prepare for a forward declaration to be emitted
during gen_visit_struct(), so that a future patch can switch
from using visit_type_FOO_implicit() to the simpler
visit_type_FOO_fields() as part of unboxing the base class
of a struct.

No change to generated code.

Signed-off-by: Eric Blake <[email protected]>
Message-Id: <1445898903[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>