[llvm] [llvm-cov][CoverageView] minor fix/improvement to HTML and text coverage output (PR #80952)

[Wentao Zhang via llvm-commits]

whentojump wrote:

I can confirm these commits are breaking two tests, and specifically the HTML part:

```
  LLVM :: tools/llvm-cov/mcdc-general-none.test
  LLVM :: tools/llvm-cov/mcdc-general.test
```

I'll investigate and update tests if that's the reason for failure.

https://github.com/llvm/llvm-project/pull/80952