[lldb] Warn when Mach-O files have overlapping segments

I recently came across a binary that for some reason had overlapping
sections. When debugging it, LLDB was able to get information about one
of the sections but not the other because SectionLoadList assumes that
each address maps to exactly one section. We have the capability to warn
about this, but it was not turned on.

rdar://105751700

Differential Revision: https://reviews.llvm.org/D144528