[llvm-cov] Add SourceNames to SourceCoverageViews, NFC

A SourceName can be a file or a function. It makes sense to attach this
information to a SourceCoverageView, seeing as views (1) already point
to the text corresponding to the relevant source code and (2) are
already used to render that text along with the SourceNames.

This is a nice cleanup which is independent of the upcoming html patch.

While we're at it, document the fields in SourceCoverageView.

llvm-svn: 273634