[C++ PATCH] builtin fn creation

https://gcc.gnu.org/ml/gcc-patches/2019-10/msg01283.html
	* decl.c (builtin_function_1): Merge into ...
	(cxx_builtin_function): ... here.  Nadger the decl before maybe
	copying it.  Set the context.
	(cxx_builtin_function_ext_scope): Push to top level, then call
	cxx_builtin_function.

From-SVN: r277102