Added basic unit test for llvm-cov.

This test compares the output of llvm-cov against a coverage file
generated by gcov. Currently, llvm-cov does not work on certain
platforms (namely big-endian architectures such as PowerPC, among
others). These platforms are marked as XFAIL for now, but will be fixed
later.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194616 91177308-0d34-0410-b5e6-96231b3b80d8