[DWARF] Fix bad comparator in sortGlobalExprs.

The comparator passed to std::sort must provide a strict weak ordering;
otherwise, the behavior is undefined.

Fixes an assertion failure generating debug info for globals
split by GlobalOpt. I have a testcase, but not sure how to reduce it,
so not included here.  (Someone else came up with a testcase, but I
can't reproduce the crash with it, presumably because my version of LLVM
ends up sorting the array differently.)

This isn't really a complete fix (see the FIXME in the patch), but at
least it doesn't have undefined behavior.

Differential Revision: https://reviews.llvm.org/D38830

llvm-svn: 315619