Fix dwarf sequence insertions

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.

Reviewers: andrew.w.kaylor, clayborg

Subscribers: sas, lldb-commits

Differential Revision: http://reviews.llvm.org/D15979

Change by Francis Ricci <fjricci@fb.com>

llvm-svn: 257132