gdb: New function to open source file and compute line charpos data

gdb: New function to open source file and compute line charpos data

Every place that a symtab's line_charpos data is loaded always follows
the same pattern, so create a new function to contain this logic and
make use of it throughout GDB.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* source-cache.c (source_cache::get_plain_source_lines): Use
	open_source_file_with_line_charpos instead of just
	open_source_file, remove call to find_source_lines.
	(source_cache::get_source_lines): Likewise.
	* source.c (find_source_lines): Make static.
	(get_filename_and_charpos): Renamed into...
	(open_source_file_with_line_charpos): ..this along with changes to
	return a scoped_fd, and some other minor clean ups.
	(identify_source_line): Use open_source_file_with_line_charpos.
	(search_command_helper): Use open_source_file_with_line_charpos
	instead of just open_source_file, remove call to
	find_source_lines.
	* source.h (open_source_file_with_line_charpos): Declare new
	function.
	(find_source_lines): Delete declaration.