[llvm-cov] Use less space to describe source names

[llvm-cov] Use less space to describe source names

In r279628, we made SourceCoverageView list the binary associated with a
view and started adding labels (e.g "Source: foo" or "Function: bar") to
everything. Condense this information a bit to unclutter reports.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280896 91177308-0d34-0410-b5e6-96231b3b80d8