Move pass to bind_back.verify.cpp and add better comment

Addresses #81055 (comment)

Author: JMazurkiewicz

libcxx/test/std/utilities/function.objects/func.bind.partial/bind_back.verify.cpp

@@ -17,11 +17,15 @@
 
 #include "types.h"
 
+constexpr int pass(int n) { return n; }
+
 void test() {
-  { // Various failures
-    auto p = std::bind_back(pass, 1);
-    static_assert(p() == 1); // expected-error {{static assertion expression is not an integral constant expression}}
+  { // Test calling constexpr function from non-constexpr `bind_back` unspecified-type
+    auto f1 = std::bind_back(pass, 1);
+    static_assert(f1() == 1); // expected-error {{static assertion expression is not an integral constant expression}}
+  }
 
+  { // Various failures
     auto d = std::bind_back(do_nothing, 2); // expected-error {{no matching function for call to 'bind_back'}}
   }
 

libcxx/test/std/utilities/function.objects/func.bind.partial/types.h

@@ -35,8 +35,6 @@ struct CopyMoveInfo {
   constexpr CopyMoveInfo(CopyMoveInfo&&) : copy_kind(move) {}
 };
 
-constexpr int pass(int n) { return n; }
-
 template <class T>
 T do_nothing(T t) {
   return t;