[lldb][ELF] Add a comment to explain address class map type

It was pointed out that ordering is crucial here, so note that.

I also looked into using a vector instead, as described in
https://llvm.org/docs/ProgrammersManual.html#dss-sortedvectorset.

Which this is in theory perfect for, but we have at least 2 places
that update the map and both would need to sort/unique each time.
Plus this code is pretty bug prone.

If there is future refactoring it's one thing to consider.

Author: DavidSpickett

lldb/source/Plugins/ObjectFile/ELF/ObjectFileELF.h

@@ -187,6 +187,9 @@ class ObjectFileELF : public lldb_private::ObjectFile {
   typedef DynamicSymbolColl::iterator DynamicSymbolCollIter;
   typedef DynamicSymbolColl::const_iterator DynamicSymbolCollConstIter;
 
+  /// An ordered map of file address to address class. Used on architectures
+  /// like Arm where there is an alternative ISA mode like Thumb. The container
+  /// is ordered so that it can be binary searched.
   typedef std::map<lldb::addr_t, lldb_private::AddressClass>
       FileAddressToAddressClassMap;