core: add tree-node comments

core: add tree-node comments

As we've moved to 64-bit systems, the padding information has become
conditionally inaccurate.  I also hit cases where invalid tree codes
did not get flagged as invalid.

	gcc/
	* tree-core.h (tree_decl_with_vis, tree_function_decl):
	Note additional padding on 64-bits
	* tree.c (cache_integer_cst): Note why no caching of enum literals.
	(get_tree_code_name): Robustify error case.