gdb: Make ldirname return a std::string

gdb: Make ldirname return a std::string

Eliminates several uses of cleanups.

Tested on x86_64 Fedora 23 with Python 2 and 3.

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

	* dwarf2read.c (struct file_and_directory): New.
	(dwarf2_get_dwz_file): Adjust to use std::string.
	(dw2_get_file_names_reader): Adjust to use file_and_directory.
	(find_file_and_directory): Adjust to return a file_and_directory
	object.
	(read_file_scope): Adjust to use file_and_directory.  Remove
	make_cleanup/do_cleanups calls.
	(open_and_init_dwp_file): Adjust to use std::string.  Remove
	make_cleanup/do_cleanups calls.
	* python/python.c (do_start_initialization): Adjust to ldirname
	returning a std::string.
	* utils.c (ldirname): Now returns a std::string.
	* utils.h (ldirname): Change return type to std::string.
	* xml-syscall.c (xml_init_syscalls_info): Adjust to ldirname
	returning a std::string.
	* xml-tdesc.c (file_read_description_xml): Likewise.