[llvm-cov] Allow commas in filenames passed to `-object` flag

Currently, -object takes a comma separated list of objects as an
argument, which prevents it working with path names that contain a
comma. Drop comma-separated support, which requires to set pass the
-object flag multiple times to set multiple objects.

Patch by Andrew Gallagher!

Differential Revision: https://reviews.llvm.org/D87003