re PR c++/71828 (ICE on valid C++11 code with constexpr __Complex int variable declaration: in operand_equal_p, at fold-const.c:2790)

	PR c++/71828
	* constexpr.c (cxx_eval_constant_expression) <case REALPART_EXPR>:
	For lval don't use cxx_eval_unary_expression and instead recurse
	and if needed rebuild the reference.

	* g++.dg/cpp0x/constexpr-71828.C: New test.

From-SVN: r238442