[llvm-cov] Emit a summary in the report directory's index

[llvm-cov] Emit a summary in the report directory's index

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281011 91177308-0d34-0410-b5e6-96231b3b80d8