Validate that destination gcov file does not exist for gcov-tool (PR gcov-profile/78783).

2017-04-18  Martin Liska  <mliska@suse.cz>

	PR gcov-profile/78783
	* libgcov-driver.c (gcov_get_filename): New function.
2017-04-18  Martin Liska  <mliska@suse.cz>

	PR gcov-profile/78783
	* gcov-tool.c (gcov_output_files): Validate that destination
	file is either removed by the tool or by a user.

From-SVN: r246961