[lldb][nfc] Refactor methods with out parameter

Currently, the method `GetAttributeAddressRanges` takes a DWARFRangeList as a
parameter, just to immediately clear it. The method also returns the size of
this list. Such an API was obfuscating the intent of the call sites (it's not
clear from the method name what it returns) and it was obfuscating redundant
checks on the size of the list.

This commit refactors the method to return the list and to also make the call
sites use the more explicit `IsEmpty` method.

Differential Revision: https://reviews.llvm.org/D151451