[llvm-cov] Add lcov tracefile export format.

Summary:
lcov tracefiles are used by various coverage reporting tools and build
systems (e.g., Bazel). It is a simple text-based format to parse and
more convenient to use than the JSON export format, which needs
additional processing to map regions/segments back to line numbers.

It's a little unfortunate that "text" format is now overloaded to refer
specifically to JSON for export, but I wanted to avoid making any
breaking changes to the UI of the llvm-cov tool at this time.

Patch by Tony Allevato (@allevato).

Reviewers: Dor1s, vsk

Reviewed By: Dor1s, vsk

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D54266

llvm-svn: 346506