c++: Rename DECL_BUILTIN_P to DECL_UNDECLARED_BUILTIN_P

c++: Rename DECL_BUILTIN_P to DECL_UNDECLARED_BUILTIN_P

I realized I'd misnamed DECL_BUILTIN_P, it's only true of compiler
builtins unless and until the user declares them -- at that point
they're real decls, and will have a location in the user's source.
(BUILT_IN_FN and friends still work though).  This renames them so
future-me is not confused as to why the predicate becomes false.

	gcc/cp/
	* cp-tree.h (DECL_BUILTIN_P): Rename to ...
	(DECL_UNDECLARED_BUILTIN_P): ... here.
	* decl.c (duplicate_decls): Adjust.
	* name-lookup.c (anticipated_builtin_p): Adjust.
	(do_nonmember_using_decl): Likewise.
	libcc1/
	* libcp1plugin.cc (supplement_binding): Rename
	DECL_BUILTIN_P.