llvm-cov: Expect a source file as a positional parameter

llvm-cov: Expect a source file as a positional parameter

Currently, llvm-cov isn't command-line compatible with gcov, which
accepts a source file name as its first parameter and infers the gcno
and gcda file names from that. This change keeps our -gcda and -gcno
options available for convenience in overriding this behaviour, but
adds the required parameter and inference behaviour as a compatible
default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200417 91177308-0d34-0410-b5e6-96231b3b80d8