Avoid compile error by giving the test type a user defined default constructor

llvm-svn: 280780

libcxx/test/std/experimental/optional/optional.object/optional.object.assign/assign_value.pass.cpp

@@ -20,7 +20,7 @@
 using std::experimental::optional;
 
 struct AllowConstAssign {
-  AllowConstAssign() = default;
+  AllowConstAssign() {}
   AllowConstAssign(AllowConstAssign const&) {}
   AllowConstAssign const& operator=(AllowConstAssign const&) const {
       return *this;