d: Factor d_nested_class and d_nested_struct into single function.

Both do the exact same operation, just on different AST nodes.

gcc/d/ChangeLog:

	* d-codegen.cc (d_nested_class): Rename to ...
	(get_outer_function): ... this.  Handle all aggregate declarations.
	(d_nested_struct): Remove.
	(find_this_tree): Use get_outer_function.
	(get_framedecl): Likewise.