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
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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200418 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-cov: Accept the long forms of gcov options

This is a bit imperfect, as these options don't show up in the help as
is and single dash variants are accepted, which differs from gcov.
Unfortunately, this seems to be as good as it gets with the cl::opt
machinery, so it'll do as an incremental step.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200419 91177308-0d34-0410-b5e6-96231b3b80d8