[Serialization] The hash seed for DeclCommentTableInfo can't change

Unlike compiled modules, swiftdoc files are considered a stable
format, so we can't change how information is stored in them. If we
add any more string-hashed tables to swiftdoc files, we should
consider using a new hash seed for those.

lib/Serialization/ModuleFile.cpp

@@ -972,8 +972,7 @@ public:
 
   hash_value_type ComputeHash(internal_key_type key) {
     assert(!key.empty());
-    // FIXME: DJB seed=0, audit whether the default seed could be used.
-    return llvm::djbHash(key, 0);
+    return llvm::djbHash(key, SWIFTDOC_HASH_SEED_5_1);
   }
 
   static bool EqualKey(internal_key_type lhs, internal_key_type rhs) {