c++: Fix rejects-valid bug in cxx_eval_outermost_constant_expr [PR93905]

Add testcase for a bug that has been just on the 8 branch.

2020-02-26  Jakub Jelinek  <jakub@redhat.com>

	PR c++/93905
	* g++.dg/cpp0x/pr93905.C: New test.

gcc/testsuite/ChangeLog

+2020-02-26  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/93905
+	* g++.dg/cpp0x/pr93905.C: New test.
+
 2020-02-25  Mihail Ionescu  <mihail.ionescu@arm.com>
 
 	* gcc.target/aarch64/advsimd-intrinsics/bf16_vstn.c: New test.

gcc/testsuite/g++.dg/cpp0x/pr93905.C

+// PR c++/93905
+// { dg-do compile { target c++11 } }
+
+enum class E { VALUE };
+
+struct B {
+  E e{E::VALUE};
+protected:
+  ~B () = default;  
+};
+
+struct D : B {};
+
+int
+main ()
+{
+  D d{};
+}