qapi: Permit flat union members for any tag value

qapi: Permit flat union members for any tag value

Flat union branch names match the tag enum's member names.  Omitted
branches default to "no members for this tag value".

Branch names starting with a digit get rejected like "'data' member
'0' has an invalid name".  However, omitting the branch works.

This is because flat union tag values get checked twice: as enum
member name, and as union branch name.  The former accepts leading
digits, the latter doesn't.

Branches whose names start with a digit therefore cannot have members.
Feels wrong.  Get rid of the restriction by skipping the latter check.

This can expose c_name() to input it can't handle: a name starting
with a digit.  Improve it to return a valid C identifier for any
input.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210323094025.3569441-9-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Commit message rewritten]