gcov: add info about "calls" to JSON output format

gcov: add info about "calls" to JSON output format

gcc/ChangeLog:

	* doc/gcov.texi: Document the new "calls" field and document
	the API bump. Mention also "block_ids" for lines.
	* gcov.cc (output_intermediate_json_line): Output info about
	calls and extend branches as well.
	(generate_results): Bump version to 2.
	(output_line_details): Use block ID instead of a non-sensual
	index.

gcc/testsuite/ChangeLog:

	* g++.dg/gcov/gcov-17.C: Add call to a noreturn function.
	* g++.dg/gcov/test-gcov-17.py: Cover new format.
	* lib/gcov.exp: Add options for gcov that emit the extra info.