[llvm-cov] Generate "report" for given source paths if sources are specified.

Summary:
Documentation says that user can specify sources for both "show" and
"report" commands. "Show" command respects specified sources, but "report" does
not. It is useful to have both "show" and "report" generated for specified
sources. Also added tests to for both commands with sources specified.

Reviewers: vsk, kcc

Reviewed By: vsk

Differential Revision: https://reviews.llvm.org/D38860

llvm-svn: 315685