Fix a bug where an error was emitted for GCC union types. (#159401)

GCC doesn't add DW_AT_data_member_location attributes to the
DW_TAG_member children of DW_TAG_union_type types. An error was being
emitted incorrectly for these cases fr om the DWARFASTParserClang. This
fixes that issue and adds a test.

GitOrigin-RevId: 370db9c62910195e664e82dde6f0adb3e255a4fd