llvm-cov: Support gcov's extermely lenient treatment of -o

In gcov, the -o flag can accept either a directory or a file name.
When given a directory, the gcda and gcno files are expected to be in
that directory. When given a file, the gcda and gcno files are
expected to be named based on the stem of that file. Non-existent
paths are treated as files.

This implements compatible behaviour.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201555 91177308-0d34-0410-b5e6-96231b3b80d8