[llvm-cov] Add an option for "export" command to emit only file summary data.

Summary:
That allows to get the same data as produced by "llvm-cov report",
but in JSON format, which is better for further processing by end users.

Reviewers: vsk

Reviewed By: vsk

Differential Revision: https://reviews.llvm.org/D41085

llvm-svn: 320435