[llvm-cov] Rename SourceCoverageView::LineCoverageInfo to LineCoverageStats, NFC

Pull LineCoverageInfo out of SourceCoverageView and rename it so that it
doesn't conflict with another class of the same name in
CoverageSummaryInfo.h.

This cuts down on the amount of code we have to move into a `protected`
section of SourceCoverageView for the upcoming html patch. It also makes
the code a bit clearer: having two LineCoverageInfo's is strange.

llvm-svn: 273633