re PR c++/84942 (internal compiler error: in fold_convert_const_int_from_real, at fold-const.c:2011)

	PR c++/84942
	* pt.c (tsubst_copy_and_build) <case FIX_TRUNC_EXPR>: Replace
	cp_build_unary_op call with gcc_unreachable ().

	* g++.dg/cpp1y/pr84942.C: New test.

From-SVN: r258821