[llvm-cov] Add support for exporting coverage data to JSON

[llvm-cov] Add support for exporting coverage data to JSON

This enables users to export coverage information as portable JSON for use by
analysis tools and storage in document based databases.

The export sub-command is invoked just like the others:

  llvm-cov export -instr-profile path/to/foo.profdata path/to/foo.binary

The resulting JSON contains a list of files and functions. Every file object
contains a list of segments, expansions, and a summary of the file's region,
function, and line coverage. Every function object contains the function's name
and regions. There is also a total summary for the entire object file.

Patch by Eddie Hurtig!

Differential Revision: https://reviews.llvm.org/D22651

llvm-svn: 276813