Improve efficiency of copying section from another tree

Improve efficiency of copying section from another tree

gcc/
	* cgraph.h (symtab_node::set_section_for_node): Declare new
	overload.
	(symtab_node::set_section_from_string): Rename from set_section.
	(symtab_node::set_section_from_node): Declare.
	* symtab.c (symtab_node::set_section_for_node): Define new
	overload.
	(symtab_node::set_section_from_string): Rename from set_section.
	(symtab_node::set_section_from_node): Define.
	(symtab_node::set_section): Call renamed set_section_from_string.
	(symtab_node::set_section): Call new set_section_from_node.