stage0: fix enum_literal to store string index, not djb2 hash

zirEnumLiteral was storing simpleStringHash(name) — a djb2 hash — as the
enum_literal IP key field. Upstream Zig stores a NullTerminatedString index
(getOrPutString). All three read sites treat the field as a string table
index and dereference it into ip->string_bytes, so the hash value caused
silent out-of-bounds reads whenever enum_literal coercion, callconv, or
signedness parsing were exercised.

Replace simpleStringHash with ipGetOrPutString, matching upstream exactly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

stage0/sema.c

@@ -17,16 +17,6 @@
         }                                                                     \
     } while (0)
 
-// Simple djb2 hash for enum literal names.
-static uint32_t simpleStringHash(const char* s) {
-    uint32_t h = 5381;
-    while (*s) {
-        h = h * 33 + (uint8_t)*s;
-        s++;
-    }
-    return h;
-}
-
 #define SEMA_AIR_INITIAL_CAP 256
 #define SEMA_AIR_EXTRA_INITIAL_CAP 256
 #define INST_MAP_INITIAL_CAP 32
@@ -9976,7 +9966,7 @@ static AirInstRef zirEnumLiteral(Sema* sema, uint32_t inst) {
     InternPoolKey key;
     memset(&key, 0, sizeof(key));
     key.tag = IP_KEY_ENUM_LITERAL;
-    key.data.enum_literal = simpleStringHash(name);
+    key.data.enum_literal = ipGetOrPutString(sema->ip, name);
     return AIR_REF_FROM_IP(ipIntern(sema->ip, key));
 }