[llvm-cov] Implement -ignore-filename-regex= option for excluding source files.

Summary:
The option is helpful for large projects where it's not feasible to specify sources which
user would like to see in the report. Instead, it allows to black-list specific sources via
regular expressions (e.g. now it's possible to skip all files that have "test" in its name).

This also partially fixes https://bugs.llvm.org/show_bug.cgi?id=34277

Reviewers: vsk, morehouse, liaoyuke

Reviewed By: vsk

Subscribers: kcc, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D43907

llvm-svn: 329581