[llvm-cov] Add support for -skip-functions to lcov

Summary:
This flag was added for the json format to exclude functions from the
output. This mirrors that behavior in lcov (where it was previously
accepted but ignored). This makes the output file smaller which can be
beneficial depending on how you consume it, especially if you don't use
this data anyways.

Patch by Keith Smiley (@keith).

Reviewers: kastiglione, Dor1s, vsk, allevato

Reviewed By: Dor1s, allevato

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73160