CC: @slavapestov

This has size wins for android and Darwin as well, since the metadata here can be synthesized at runtime. CC: @jckarter

Definitely, it'd be awesome to emit just a minimal symbolic representation of the CF type and build the metadata structure on demand. On Darwin, however, we will need to maintain support for the old mechanism for the now-stable ABI, though.

Hmm, what would be the best way to do this now then?

On Darwin, the runtime would need to be able to continue to provide the existing `swift_getForeignTypeMetadata` entry point and accept the current format of pre-generated foreign metadata records. Either you could add a new entry point that uses minimal templates, or maybe you could have the one entry point recognize from the format of the arguments whether it's an old-format metadata record or a new-format minimal record. This might be possible because an existing metadata record should only ever have the value of MetadataKind::Struct, ::Enum, or ::ForeignClass as a pointer-sized int value at the address point, whereas if you instead passed in a nominal type descriptor with foreign instantiation information or something like that, it should never have one of those values at the address point because of additional flags.

#22857