c++: Add lang_decl, type_decl API

c++: Add lang_decl, type_decl API

We need to call the lang_decl and type_decl creators from the module
loading machinery.  This makes them reachable.

	gcc/cp/
	* cp-tree.h (maybe_add_lang_decl_raw, maybe_add_lang_type_raw):
	Declare.
	* lex.c (maybe_add_lang_decl_raw, maybe_add_lang_type_raw):
	Externalize, reformat.