llvm-cov: Implement --no-output

llvm-cov: Implement --no-output

In gcov, there's a -n/--no-output option, which disables the writing
of any .gcov files, so that it emits only the summary info on stdout.
This implements the same behaviour in llvm-cov.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208148 91177308-0d34-0410-b5e6-96231b3b80d8