llvm-cov: Combine segments that cover the same location

If we have multiple coverage counts for the same segment, we need to
add them up rather than arbitrarily choosing one. This fixes that and
adds a test with template instantiations to exercise it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218432 91177308-0d34-0410-b5e6-96231b3b80d8