[lldb/DWARF] Add support for location lists in package files

The only thing needed was to account for the offset from the
debug_cu_index section when searching for the location list.

This patch also fixes a bug in the Module::ParseAllDebugSymbols
function, which meant that we would only parse the variables of the
first compile unit in the module. This function is only used from
lldb-test, so this does not fix any real issue, besides preventing me
from writing a test for this patch.