Hard-code a random constant as upb's hash seed

This is a step toward randomizing upb's map ordering, which is currently
deterministic.

PiperOrigin-RevId: 721030827

upb/hash/common.c

@@ -402,8 +402,12 @@ uint32_t _upb_Hash(const void* p, size_t n, uint64_t seed) {
   return Wyhash(p, n, seed, kWyhashSalt);
 }
 
+// Returns a seed for upb's hash function. For now this is just a hard-coded
+// constant, but we are going to randomize it soon.
+static uint64_t _upb_Seed(void) { return 0xdff497e08889bf61; }
+
 static uint32_t _upb_Hash_NoSeed(const char* p, size_t n) {
-  return _upb_Hash(p, n, 0);
+  return _upb_Hash(p, n, _upb_Seed());
 }
 
 static uint32_t strhash(upb_tabkey key) {