Revert "[ASTImporter] Fix crash caused by unset AttributeSpellingListIndex"

This broke the GreenDragon build, due to the following error while running
TestImportBuiltinFileID:

```
Ignored/unknown shouldn't get here
UNREACHABLE executed at tools/clang/include/clang/Sema/AttrSpellingListIndex.inc:13!
```

See http://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/24213/

This reverts commit 73c6beb2.
This reverts https://reviews.llvm.org/D89318