llvm-cov: Support specifying multiple source files

Make llvm-cov compatible with gcov for cases where multiple files are
specified on the command line. That is, loop over each one and report
coverage, and report errors on stderr only rather than via return
code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211959 91177308-0d34-0410-b5e6-96231b3b80d8