Allow users of the GCOV API to extend the FileInfo class to implement custom output formats

Summary:
The GCOV API can be used to parse gcda/gcno files, but in order to implement custom output formats, users need to reimplement everything.
If the FileInfo members were protected instead of private, they'd be able to reuse the code.

Reviewers: bogner, davide, scott.smith

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41802

llvm-svn: 338013