[llvm-cov] Use a stable sort on sub-views

We need to use a stable sort on instantiation and expansion sub-views to
produce consistent output. Fortunately, we've gotten lucky and the tests
have checks for the stable order.

This is needed to unblock D39245. Once that lands, we'll have better
test coverage for sort non-determinism.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316490 91177308-0d34-0410-b5e6-96231b3b80d8