[sancov] introducing symbolized coverage files (.symcov)

Summary:
Answering any meaningful questions about .sancov files requires
accessing symbol information from the corresponding binary.

This change introduces a separate intermediate data structure and
format: symbolized coverage. It contains all symbol information that
is required to answer common queries:
- merging
- coverd/uncovered files and functions
- line status.

Also removing the html report functionality from sancov: generated
HTML files are too huge, and a different approach is required.
Maintaining this half-working approach in the C++ is painful.

Differential Revision: https://reviews.llvm.org/D24947

llvm-svn: 282639