llvm-cov: move the gcov code into a separate file.

The gcov compatible code is moved to its own file and
llvm-cov is updated to be a wrapper that always calls
the gcov main function.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214107 91177308-0d34-0410-b5e6-96231b3b80d8