Allow html coverage reports to use absolute hyperlinks

[mightybyte]

I would like to store HTML coverage reports generated by llvm-cov in non-publicly accessible Google Cloud Storage bucket. When I do this, GCS rewrites the URLs when the page loads. This breaks the coverage report links and makes the report much less useful. It would be nice if there was a way to make the HTML coverage reports use absolute links with a user-specified prefix that allows the coverage reports to work properly when used in this kind of a link rewriting environment.