Add support for DW_AT_export_symbols for anonymous structs

Summary:
We add support for DW_AT_export_symbols to detect anonymous struct on top of the heuristics implemented in D66175
This should allow us to differentiate anonymous structs and unnamed structs.
We also fix TestTypeList.py which was incorrectly detecting an unnamed struct as an anonymous struct.

Differential Revision: https://reviews.llvm.org/D68961