Added basic unit test for llvm-cov.

This test compares the output of llvm-cov against a coverage file
generated by gcov. Since the source file must be in the current
directory when reading GCNO files, the test will first cd into the
Inputs directory.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194451 91177308-0d34-0410-b5e6-96231b3b80d8