[lldb][NFCI] Refactor out attribute parsing from DWARFASTParserClang::ParseSingleMember

[lldb][NFCI] Refactor out attribute parsing from DWARFASTParserClang::ParseSingleMember

D68422 introduced `ParsedDWARFTypeAttributes` which encapsulated attribute
parsing and storage into its own small struct. This patch is doing the same for
the member type attribute parsing. One utility class is parsing normal member
attributes and the other is parsing the dedicated Objective-C property
attributes.

Right now the patch just makes the `ParseSingleMember` function a bit shorter,
but the bigger benefit is that we can now split up the function into Objective-C
property parsing and parsing of normal members (struct/class members and
Objective-C ivars). The only shared code between those two parsing logic is the
normal member attribute parsing.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D111494