* constexpr.c (literal_type_p): Return true for void type in C++14.

	* g++.dg/cpp0x/constexpr-function2.C: Limit dg-error to C++11.
	* g++.dg/cpp0x/constexpr-neg1.C: Likewise.
	* g++.dg/cpp1y/constexpr-void1.C: New test.

From-SVN: r218220