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

When all provided source files are filtered out either due to `--ignore-filename-regex` or not part of binary, don't generate coverage reults for all source files. Because if users want to generate coverage results for all source files, they don't even need to provid selected source files or `--ignore-filename-regex`.

Differential Revision: https://reviews.llvm.org/D89359