This fixes a segfault in translate-c that would previously occur when
initializing structs with unnamed bitfields, due to a failed assertion in
`transInitListExprRecord`. Unnamed bitfields do not have initializers,
so `transInitListExprRecord` erroneously assumes that `init_count` equals
the number of fields in the record.
Since attempting to initialize an opaque type is a syntax error in Zig,
we can just demote any attempts to initialize them.