parser,ast,cgen: support nested unions with field union {, to improve c interoperability (similar to field struct {)
#20097
| Job | Run time |
|---|---|
| 7s | |
| 2m 5s | |
| 1m 16s | |
| 7m 38s | |
| 4m 21s | |
| 15m 27s |