Change llvm-cov output formatting to be more similar to gcov.

- Replaced tabs with proper padding
- print() takes two arguments, which are the GCNO and GCDA filenames
- Files are listed at the top of output, appended by line 0
- Stripped strings of trailing \0s
- Removed last two lines of whitespace in output

Patch by Yuchen Wu!

llvm-svn: 193148