Unnamed union fields with enums are checked for, but if `find_field`
causes an ICE then the compiler won't get to that point.
Signed-off-by: David Wood <david@davidtw.co>
- improve diagnostics of field uniqueness check and representation check
- simplify the implementation of field uniqueness check
- remove some useless codes and improvement neatness
Anonymous structs or unions are only allowed in struct field
definitions.
Co-authored-by: carbotaniuman <41451839+carbotaniuman@users.noreply.github.com>