[llvm-cov] Add the ability to specify directories of input source files

[llvm-cov] Add the ability to specify directories of input source files

We've supported restricting coverage reports to a set of files for a
long time. Add support for being able to restrict by entire directories.

I suppose this supersedes D20803.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282202 91177308-0d34-0410-b5e6-96231b3b80d8