re PR c++/47783 (Warning 'set but not used' [-Wunused-but-set-parameter] incorrectly issued for update through reference wrapper)

	PR c++/47783
	* cvt.c (convert_from_reference): Call mark_exp_read.

	* g++.dg/warn/Wunused-parm-4.C: New test.

From-SVN: r170255