Fix dwarf sequence insertions

[fjricci]

Summary

If two dwarf sequences begin with entries that have identical addresses,
it is possible for the comparator to order the first entry of the new
sequence between the first and second entries of the existing sequence.

This will result in an attempted insertion of the second sequence inside
of the first sequence, which is invalid.

Ensure that insertions only occur in between existing sequences.

[clayborg]

Looks good.

[sas]

Committed as r257132.

source/Symbol/LineTable.cpp

if (m_entries.empty() || !Entry::EntryAddressLessThan(entry, m_entries.back()))
         return;
     }
 
     // Otherwise, find where this belongs in the collection
     entry_collection::iterator begin_pos = m_entries.begin();
     entry_collection::iterator end_pos = m_entries.end();
     LineTable::Entry::LessThanBinaryPredicate less_than_bp(this);
     entry_collection::iterator pos = upper_bound(begin_pos, end_pos, entry, less_than_bp);
+
+    // We should never insert a sequence in the middle of another sequence
+    if (pos != begin_pos) {
+        while (pos < end_pos && !((pos - 1)->is_terminal_entry))
+            pos++;
+    }
+
 #ifdef LLDB_CONFIGURATION_DEBUG
     // If we aren't inserting at the beginning, the previous entry should
     // terminate a sequence.
     if (pos != begin_pos)
     {
         entry_collection::iterator prev_pos = pos - 1;
         assert(prev_pos->is_terminal_entry);
     }