translate-c: demote structs with opaque fields to opaque

[thonkinator]

Uses the same logic as demoting structs with bitfields. I also noticed that Translator.alignmentForField panics when given a bitfield, which could be replicated for opaque fields, but I figured the repeated call to Translator.typeWasDemotedToOpaque was not preferable.

Fixes ziglang/translate-c#182

[alexrp]

https://github.com/ziglang/translate-c is the canonical repository for translate-c, please submit this PR there.

[thonkinator]

@alexrp that repository doesn't seem to be used for the zig compiler, does that mean that there is no way to contribute to the translate-c that is actually used in the compiler? i've seen other accepted pull requests to this repository fixing issues on that one

[alexrp]

We will be periodically synchronizing https://github.com/ziglang/translate-c into this repository, at least until #20630 happens.

The PRs you've seen were probably from before #24497. After that PR, the only translate-c code that's maintained in this repository is glue code; the core translation logic is maintained in https://github.com/ziglang/translate-c.