[lldb][NFC] Early-exit in DWARFASTParserClang::ParseSingleMember

ParseSingleMember has two large ifs around the back of it's body:
`if (!is_artificial)` and `if (member_type)`. This patch just converts those
to early-exits. The patch is NFC. It even retains the curious fact that
Objective-C properties that fail to parse are silently ignored, but now there
is at least a FIXME that points this out.