[lldb][DWARFASTParserClang] Make MemberAttributes const when parsing member DIEs (#71921)

This patch removes the Objective-C accessibility workaround added in
https://github.com/llvm/llvm-project/commit/5a477cfd90a8a12510518973fa450ae67372f199
(rdar://8492646). This allows us to make the local `MemberAttributes`
variable immutable, which is useful for some other work around this
function I was planning on doing.

We don't need the workaround anymore since compiler-support for giving
debuggers access to private ivars was done couple of years later in
https://github.com/llvm/llvm-project/commit/d6cb4a858db0592f6f946fd99a10a9dfcbea6ee9
(rdar://10997647).

**Testing**

* Test-suite runs cleanly