llvm-cov: Removed output to STDOUT/specified file.

Instead of asking the user to specify a single file to output coverage
info and defaulting to STDOUT, llvm-cov now creates files for each
source file with a naming system of: <source filename> + ".llcov".

This is what gcov does and although it can clutter the working directory
with numerous coverage files, it will be easier to hook the llvm-cov
output to tools which operate on this assumption (such as lcov).

llvm-svn: 196184