[cxx-interop] Do not crash when importing anonymous classes
This was discovered during interop adoption in SwiftCompilerSources.
```
/Volumes/Projects/swift/swift/include/swift/SIL/SILNode.h:180:5 <Spelling=/Volumes/Projects/swift/swift/include/swift/SIL/SILNode.h:171:3>: while adding SwiftName lookup table entries for clang declaration 'swift::SILNode::SharedUInt8Fields::(anonymous)'
```
