Fix reference-type struct and array fields in the wast parser

[aizu-m]

Debug wat2wasm, --enable-gc:

type.h:75: Assertion failed: (!IsReferenceWithIndex()), function Type

Found while fuzzing the text parser with all features enabled. A struct or array field whose type is a reference, such as (ref 0), (ref $t) or (ref null $t), reaches ParseField. It built the field type with Type(type.opt_type()). opt_type() only carries the Type::Enum, so the reference index is dropped and the single-argument Type(Enum) constructor asserts. A release build skips the assert and keeps a wrong type (index 0).

Every other field and value-type site already routes through VarToType (ParseGlobalType, the table and elem paths), which keeps the index and defers named references. ParseField now does the same. VarToType stores a pointer into the field for end-of-module resolution, and ParseFieldList copies each field into a vector, so those pointers are repointed at the stored elements once the vector has stopped growing.

Test covers indexed, named backward, named forward, mut and abstract-ref fields in both struct and array.

[zherczeg]

GC patches reworks these, probably not worth to update the code before those patches land.

[aizu-m]

Fair enough. I had a look at #2607: it makes the same change in ParseField (VarToType(type, &field->type)) and moves ParseFieldList over to a StructType*, so the per-field vector copy my patch had to repoint around disappears as well. No point duplicating that.

Happy to leave it to the GC series. The test here covers indexed, named backward and named forward refs in both struct and array, so take it across if it's useful coverage; otherwise this can just be closed.