RISC-V: Add const to function_shape::get_name [NFC]

function_shape::get_name is the funciton for building intrinsic function name,
the result should not be changed by others once it built.

So add const to the return type to make sure no one change that by
accident.

gcc/ChangeLog:
	* config/riscv/riscv-vector-builtins-shapes.cc
	(vsetvl_def::get_name): Adjust return type.
	(loadstore_def::get_name): Ditto.
	(indexed_loadstore_def::get_name): Ditto.
	(th_loadstore_width_def::get_name): Ditto.
	(th_indexed_loadstore_width_def::get_name): Ditto.
	(alu_def::get_name): Ditto.
	(alu_frm_def::get_name): Ditto.
	(widen_alu_frm_def::get_name): Ditto.
	(narrow_alu_frm_def::get_name): Ditto.
	(reduc_alu_frm_def::get_name): Ditto.
	(widen_alu_def::get_name): Ditto.
	(no_mask_policy_def::get_name): Ditto.
	(return_mask_def::get_name): Ditto.
	(narrow_alu_def::get_name): Ditto.
	(move_def::get_name): Ditto.
	(mask_alu_def::get_name): Ditto.
	(reduc_alu_def::get_name): Ditto.
	(th_extract_def::get_name): Ditto.
	(scalar_move_def::get_name): Ditto.
	(vundefined_def::get_name): Ditto.
	(misc_def::get_name): Ditto.
	(vset_def::get_name): Ditto.
	(vcreate_def: Ditto.::get_name): Ditto.
	(read_vl_def::get_name): Ditto.
	(fault_load_def::get_name): Ditto.
	(vlenb_def::get_name): Ditto.
	(seg_loadstore_def::get_name): Ditto.
	(seg_indexed_loadstore_def::get_name): Ditto.
	(seg_fault_load_def::get_name): Ditto.
	(crypto_vv_def::get_name): Ditto.
	(crypto_vi_def::get_name): Ditto.
	(crypto_vv_no_op_type_def::get_name): Ditto.
	(sf_vqmacc_def::get_name): Ditto.
	(sf_vqmacc_def::get_name): Ditto.
	(sf_vfnrclip_def::get_name): Ditto.
	* config/riscv/riscv-vector-builtins.cc
	(function_builder::add_unique_function): Adjust the type for the
	function name holder.
	(function_builder::add_overloaded_function): Ditto.
	* config/riscv/riscv-vector-builtins.h (function_shape::get_name): Add
	const to the return type.