iOS15 beta3(19A5297e) crash超高

写在最前面:
注意看讨论和前后文，有的方案后面已经讨论过是有问题的，谨慎为之

```swift
0 libswiftCore.dylib 0x0000000184f86000 + 3735504
1 libswiftCore.dylib swift_checkMetadataState + 44
2 libswiftCore.dylib 0x0000000184f86000 + 3925676
3 libswiftCore.dylib 0x0000000184f86000 + 3923008
4 XXXX _propertyDescriptionsAndStartPoint (:0)
5 XXXX propertyDescriptions (Metadata.swift:214)
6 XXXX _transform (ExtendCustomModelType.swift:138)
7 XXXX _transform (ExtendCustomModelType.swift:133)
8 XXXX deserializeFrom (Deserializer.swift:78)
9 XXXX deserialize (Deserializer.swift:33)
```

这题好难

Start by reading the crash path in Deserializer.swift at lines 33 and 78, then inspect ExtendCustomModelType.swift at lines 133 and 138 and Metadata.swift at line 214. Review the discussion carefully because the issue warns that later comments reject some proposed approaches. Done means identifying and fixing the iOS 15 beta 3 crash without regressing deserialization.

Written by the indexing model from the issue text.