-Wwrite-strings: Constify macroexp.c:init_shared_buffer

-Wwrite-strings: Constify macroexp.c:init_shared_buffer

There's one call in the file that passes a string literal, like:

	init_shared_buffer (&va_arg_name, "__VA_ARGS__",
				  strlen ("__VA_ARGS__"));

Instead of adding a cast here, make init_shared_buffer take a 'const
char *', and remove the several casts in the file that are made
obsolete.

gdb/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* macroexp.c (macro_buffer::shared): Now a bool.
	(init_buffer): Update.
	(init_shared_buffer): Constify 'addr' parameter.
	(substitute_args, expand, macro_expand, macro_expand_next): Remove
	casts.