qapi: Check for QAPI collisions involving variant members

Right now, our ad hoc parser ensures that we cannot have a
flat union that introduces any members that would clash with
non-variant members inherited from the union's base type (see
flat-union-clash-member.json).  We want QAPISchemaObjectType.check()
to make the same check, so we can later reduce some of the ad
hoc checks.

We already have a map 'seen' of all non-variant members. We
still need to check for collisions between each variant type's
members and the non-variant ones.

To know the variant type's members, we need to call
variant.type.check().  This also detects when a type contains
itself in a variant, exactly like the existing base.check()
detects when a type contains itself as a base.  (Except that
we currently forbid anything but a struct as the type of a
variant, so we can't actually trigger this type of loop yet.)

Slight complication: an alternate's variant can have arbitrary
type, but only an object type's check() may be called outside
QAPISchema.check(). We could either skip the call for variants
of alternates, or skip it for non-object types.  For now, do
the latter, because it's easier.

Then we call each variant member's check_clash() with the
appropriate 'seen' map.  Since members of different variants
can't clash, we have to clone a fresh seen for each variant.
Wrap this in a new helper method
QAPISchemaObjectTypeVariants.check_clash().

Note that cloning 'seen' inside .check_clash() resembles
the one we just removed from .check() in 'qapi: Drop
obsolete tag value collision assertions'; the difference here is
that we are now checking for clashes among the qapi members of
the variant type, rather than for a single clash with the variant
tag name itself.

Note that, by construction, collisions can't actually happen for
simple unions: each variant's type is a wrapper with a single
member 'data', which will never collide with the only non-variant
member 'type'.

For alternates, there's nothing for a variant object type's
members to clash with, and therefore no need to call the new
variants.check_clash().

No change to generated code.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1447836791-369-12-git-send-email-eblake@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
