llvm-cov: Allow short options to be grouped

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207034 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-cov: Add support for gcov's --long-file-names option

GCOV provides an option to prepend output file names with the source
file name, to disambiguate between covered data that's included from
multiple sources. Add a flag to llvm-cov that does the same.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207035 91177308-0d34-0410-b5e6-96231b3b80d8