c++: Comments & formatting

c++:  Comments & formatting

I found some bad formatting and misleading or incomplete comments
during my spelunking around the c++FE.  May as well clean up trunk and
record what I noted.

	gcc/cp/
	* cp-tree.h: Correct some tree lang flag comments,
	reformat some structure definitions.  Note some structure
	sizes.  Clarify some comments.
	(yyungetc): Delete.  Not been a thing for some time.
	* class.c (copy_fndecl_with_name): Comment.
	(check_bases_and_members): Unnecessary {}.
	(layout_class_type): Comment.
	* cp-tree.def (UNBOUND_CLASS_TEMPLATE): Adjust comment.
	* decl.c:  Fix some formatting & whitespace issues.
	(function_requirements_equivalent_p): Note why
	substitutions are needed.
	* decl2.c (no_linkage_error): Note that heroics about
	'typedef struct { ... };' are no longer needed.
	* method.c: Whitespace.
	* name-lookup.c: Whitespace.
	(add_decl_to_level): Reformat a line.
	(print_binding_stack): Mark as DEBUG_FUNCTION.
	(has_using_namespace_std_directive_p): Delete comment.
	* pt.c: Whitespace
	* ptree.c: Whitespace.
	* rtti.c: Whitespace & comment.
	* tree.c: Comment.
	* typeck.c (structural_comptypes): Add comment.