YAML built_in for structs is unclear

For individual members it can be one of the SPIRV builtins. But for structures it turns into a CSV-string with ids :exploding_head:
Either it should be a proper array, or a `null`, since there are no built-in structures (I hope).

No files or tests are named. Start by locating the YAML serialization path for the `built_in` field and compare output for individual members versus structures. Done means the struct representation is no longer an ambiguous CSV string and follows the issue's agreed representation.

Written by the indexing model from the issue text.