[lldb/DWARF] Look for complete member definitions in other modules

With -flimit-debug-info, we can have a definition of a class, but no
definition for some of its members. This extends the same logic we were
using for incomplete base classes to cover incomplete members too.

Test forward-declarations.s is removed as it is no longer applicable --
we don't warn anymore when encountering incomplete members as they could
be completed elsewhere. New checks added to TestLimitDebugInfo cover the
handling of incomplete members more thoroughly.