[lldb][NFCI] Change return type of DWARFDebugInfoEntry::GetAttributes

The purpose of this method is to get the list of attributes of a
DebugInfoEntry. Prior to this change we were passing in a mutable
reference to a DWARFAttributes object and having the method fill it in
for us while returning the size of the filled out list. But
instead of doing that, we can just return a `DWARFAttributes` object
ourselves since every caller creates a new list before calling
GetAttributes.

Differential Revision: https://reviews.llvm.org/D150402