llvm-cov: Store blocks rather than counts per line.

Each line stores all the blocks that execute on that line, instead of
only storing the line counts previously accumulated. This provides more
information for each line, and will be useful for options in enabling
block and branch information.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196177 91177308-0d34-0410-b5e6-96231b3b80d8