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.
llvm-cov: Improve help message text

This Properly capitalizes and clarifies the help output from
llvm-cov. It also puts the llvm-only / non-gcov-compatible options in
their own category.