llvm-cov: Document the llvm-cov tests

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200739 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-cov: Ignore missing .gcda files

When gcov is run without gcda data, it acts as if the counts are all
zero and labels the file as - to indicate that there was no data. We
should do the same.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200740 91177308-0d34-0410-b5e6-96231b3b80d8