c++: Replace tag_scope with TAG_how

c++: Replace tag_scope with TAG_how

I always found tag_scope confusing, as it is not a scope, but a
direction of how to lookup or insert an elaborated type tag.  This
replaces it with a enum class TAG_how.  I also add a new value,
HIDDEN_FRIEND, to distinguish the two cases of innermost-non-class
insertion that we currently conflate.  Also renamed
'lookup_type_scope' to 'lookup_elaborated_type', because again, we're
not providing a scope to lookup in.

	gcc/cp/
	* name-lookup.h (enum tag_scope): Replace with ...
	(enum class TAG_how): ... this.  Add HIDDEN_FRIEND value.
	(lookup_type_scope): Replace with ...
	(lookup_elaborated_type): ... this.
	(pushtag): Use TAG_how, not tag_scope.
	* cp-tree.h (xref_tag): Parameter is TAG_how, not tag_scope.
	* decl.c (lookup_and_check_tag): Likewise.  Adjust.
	(xref_tag_1, xref_tag): Likewise. adjust.
	(start_enum): Adjust lookup_and_check_tag call.
	* name-lookup.c (lookup_type_scope_1): Rename to ...
	(lookup_elaborated_type_1) ... here. Use TAG_how, not tag_scope.
	(lookup_type_scope): Rename to ...
	(lookup_elaborated_type): ... here.  Use TAG_how, not tag_scope.
	(do_pushtag): Use TAG_how, not tag_scope.  Adjust.
	(pushtag): Likewise.
	* parser.c (cp_parser_elaborated_type_specifier): Adjust.
	(cp_parser_class_head): Likewise.
	gcc/objcp/
	* objcp-decl.c (objcp_start_struct): Use TAG_how not tag_scope.
	(objcp_xref_tag): Likewise.