[lldb] Split ParseSingleMember into Obj-C property and normal member/ivar parsing code.

Right now DWARFASTParserClang::ParseSingleMember has two parts: One part parses
Objective-C properties and the other part parses C/C++ members/Objective-C
ivars. These parts are pretty much independent of each other (with one
historical exception, see below) and in practice they parse DIEs with different
tags/attributes: `DW_TAG_APPLE_property` and `DW_TAG_member`.

I don't see a good reason for keeping the different parsing code intertwined in
a single function, so instead split out the Objective-C property parser into its
own function.

Note that 90% of this commit is just unindenting nearly all of
`ParseSingleMember` which was inside a `if (tag == DW_TAG_member)` block. I.e.,
think of the old `ParseSingleMember` function as: The rest is just moving the
property parsing code into its own function and I added the ReportError
implementation in case we fail to resolve the property type (which before was
just a silent failure).

```
lang=c++
void DWARFASTParserClang::ParseSingleMember(...) {
  [...]
  if (tag == DW_TAG_member) {
    [...] // This huge block got unindented in this patch as the `if` above is gone.
  }
  if (property) {
    [...] // This is the property parsing code that is now its own function.
  }
}
```

There is one exception to the rule that the parsers are independent. Before 2012
Objective-C properties were encoded as `DW_TAG_member` with
`DW_AT_APPLE_property*` attributes describing the property. In 2012 this has
changed in a series of commits (see for example
c0449635b35b057c5a877343b0c5f14506c7cf02 which updates the docs) so that
`DW_TAG_APPLE_property` is now used for properties. With the old format we first
created an ivar and afterwards used the `DW_AT_APPLE_property*` attributes to
create the respective property, but there doesn't seem to be any way to create
such debug info with any clang from the last 9 years. So this is technically not
NFC in case some finds debug info from that time and tries to use properties.

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D111632

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