[llvm-cov] Fix issues with segment highlighting in the html view

The text and html coverage views take different approaches to emitting
highlighted regions. That's because this problem is easier in the text
view: there's no need to worry about escaping text or adding tooltip
content to a highlighted snippet.

Unfortunately, the html view didn't get region highlighting quite right.

This patch fixes the situation, bringing parity between the two views.

llvm-svn: 280981