llvm-cov writes out an index file in '-output-dir' mode, albeit not a
very informative one. Try to fix that by using the CoverageReport API to
include some basic summary information in the index file.
llvm-svn: 281011
This makes it easy to swap out the default stylesheet for a custom one.
It also shaves ~6.62 MB out of the report directory for a full coverage
build of llvm+clang.
While we're at it, prune the CSS and add tests for it.
llvm-svn: 276359