Use an enum to represent subclasses of symbol

Use an enum to represent subclasses of symbol

This changes struct symbol to use an enum to encode the concrete
subclass of a particular symbol.  Note that "enum class" doesn't work
properly with bitfields, so a plain enum is used.

2017-11-17  Tom Tromey  <tom@tromey.com>

	* symtab.h (enum symbol_subclass_kind): New.
	(struct symbol) <is_cplus_template_function, is_rust_vtable>:
	Remove.
	<subclass>: New member.
	(SYMBOL_IS_CPLUS_TEMPLATE_FUNCTION): Update.
	* rust-lang.c (rust_get_trait_object_pointer): Update.
	* dwarf2read.c (read_func_scope): Update.
	(read_variable): Update.