[utils] Teach the code coverage prep script about --restrict

Add two options to the code coverage artifact prep script:

  * --use-existing-profdata: Use an existing indexed profile instead of
    merging the same profiles again.
  * --restrict: Restrict the coverage reporting to the given list of
    source directories.

With this in place, we can teach the coverage bot how to prepare
separate reports for each of the llvm tools.

llvm-svn: 282204