[llvm-cov] Add an API to prepare file reports (NFC)

It would be nice to prepare file reports (using the CoverageReport API)
without actually rendering them to the console. I plan on using this to
flesh out the 'index' files in the coverage views.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281009 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed