[llvm-cov] Pass LineCoverageStats in SourceCoverageView. NFC.

Instead of copying around the wrapped segment and the list of line
segments, just pass a reference to a LineCoverageStats object. This
simplifies the interface. It also makes an upcoming change to suppress
distracting highlights possible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316108 91177308-0d34-0410-b5e6-96231b3b80d8