[llvm-cov] Refactor "export" command implementation and add support for SOURCES.

Summary: Define an interface for Exporter + split JSON exporter into .h and .cpp.

Reviewers: vsk, morehouse

Reviewed By: vsk

Subscribers: llvm-commits, Dor1s, kcc

Differential Revision: https://reviews.llvm.org/D41600

llvm-svn: 321815