cgraph: Add clone_identifier function.

cgraph: Add clone_identifier function.

This is similar to clone_function_name and its siblings but takes an
identifier tree node rather than a function declaration.

This is to be used in conjunction with the identifier node stored in
cgraph_function_version_info::assembler_name to mangle FMV functions in
later patches.

gcc/ChangeLog:

	* cgraph.h (clone_identifier): New function.
	* cgraphclones.cc (clone_identifier): New function.
	(clone_function_name): Refactored to use clone_identifier.
	(is_valid_asm_symbol): New helper function.