Use move and stack allocation for RangeSpanLists. As a result make
a few things more const as well because we're now using const
references to refer to iterators.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196398 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-cov: Replace size() with empty() in bool check.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196400 91177308-0d34-0410-b5e6-96231b3b80d8