Use ui_file_as_string in gdb/compile/

Use ui_file_as_string in gdb/compile/

Using ui_file_as_string would imply changing a few prototypes to pass
around source and object file names as std::string.  Instead of that,
wrap those two in a new class.  This ends up eliminating a small
wrinkle: get_new_file_names and compile_object_load have swapped
parameters.  The former takes "source, objfile", while the latter
takes "objfile, source".

gdb/ChangeLog:
2016-11-08  Pedro Alves  <palves@redhat.com>

	* c-lang.h (c_compute_program): Now returns std::string.
	* compile/compile-internal.h (class compile_file_names): New
	class.
	* compile/compile-object-load.c (compile_object_load): Replace
	object_file and source_file parameters with a compile_file_names
	parameter.  Adjust.
	* compile-object-load.h: Include "compile-internal.h".
	(compile_object_load): Replace object_file and source_file
	parameters with a compile_file_names parameter.
	* compile/compile-c-support.c (c_compute_program): Now returns a
	std::string.  Use ui_file_as_string.
	* compile/compile.c (get_new_file_names): Remove parameters and
	return a compile_file_names instead.
	(compile_to_object): Now returns a compile_file_names.  Use
	ui_file_as_string.
	(eval_compile_command): Use compile_file_names.
	* language.h (struct language_defn) <la_compute_program>: Now
	returns std::string.