Revert "[llvm-cov] don't include all source files when provided source files are filtered out"

This reverts commit c2bd20ef652 and the follow up fix 16605bba6fb.

The tools/llvm-cov/warnings.h continues to fail on Windows platforms even
after the follow up, for example on the llvm-clang-win-x-armv7l builder:

    http://lab.llvm.org:8011/#/builders/60/builds/94