dwarf2read.c: Clean up out of bounds handling

dwarf2read.c: Clean up out of bounds handling

Multiple places in dwarf2read.c open code 1-based to 0-based index
conversion and check for out of bounds accesses to lh->include_dirs
and lh->file_names.  This commit factors those out to a couple methods
and uses them throughout.

gdb/ChangeLog:
2017-03-27  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (file_entry) <dir_index>: Add comment.
	(file_entry::include_dir): New method.
	(line_header::include_dir_at, line_header::file_name_at): New
	methods.
	(setup_type_unit_groups, setup_type_unit_groups)
	(psymtab_include_file_name): Simplify using the new methods.
	(lnp_state_machine) <the_line_header>: New field.
	<file>: Add comment.
	(lnp_state_machine::current_file): New method.
	(dwarf_record_line): Simplify using the new methods.
	(init_lnp_state_machine): Initialize the "the_line_header" field.
	(dwarf_decode_lines_1, dwarf_decode_lines, file_file_name):
	Simplify using the new methods.