Use a named rather than anon struct in instance

The anonymous struct nested in a union triggers a -Wnested-anon-type
warning ("anonymous types declared in an anonymous union are an
extension") under clang (#1204).  This names the struct and defines it
out of the definition of `instance` to get around to warning (and makes
the code slightly simpler).

include/pybind11/detail/common.h

@@ -377,16 +377,18 @@ constexpr size_t instance_simple_holder_in_ptrs() {
 struct type_info;
 struct value_and_holder;
 
+struct nonsimple_values_and_holders {
+    void **values_and_holders;
+    uint8_t *status;
+};
+
 /// The 'instance' type which needs to be standard layout (need to be able to use 'offsetof')
 struct instance {
     PyObject_HEAD
     /// Storage for pointers and holder; see simple_layout, below, for a description
     union {
         void *simple_value_holder[1 + instance_simple_holder_in_ptrs()];
-        struct {
-            void **values_and_holders;
-            uint8_t *status;
-        } nonsimple;
+        nonsimple_values_and_holders nonsimple;
     };
     /// Weak references (needed for keep alive):
     PyObject *weakrefs;