[compiler] Mark String as kNeverSerialized

Bug: v8:7790
Change-Id: I2deb462c3467f7239d55b0f295feed1de5ca1c2f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2616198
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72010}

src/compiler/heap-refs.h

@@ -64,6 +64,7 @@ enum class OddballType : uint8_t {
   /* Subtypes of String */                          \
   V(InternalizedString)                             \
   /* Subtypes of Name */                            \
+  V(String)                                         \
   V(Symbol)                                         \
   /* Subtypes of HeapObject */                      \
   V(AccessorInfo)                                   \
@@ -93,8 +94,6 @@ enum class OddballType : uint8_t {
   V(BytecodeArray)                            \
   V(FixedArray)                               \
   V(FixedDoubleArray)                         \
-  /* Subtypes of Name */                      \
-  V(String)                                   \
   /* Subtypes of JSReceiver */                \
   V(JSObject)                                 \
   /* Subtypes of HeapObject */                \

src/compiler/js-heap-broker.cc

@@ -812,7 +812,9 @@ StringData::StringData(JSHeapBroker* broker, ObjectData** storage,
       to_number_(TryStringToDouble(broker->local_isolate(), object)),
       is_external_string_(object->IsExternalString()),
       is_seq_string_(object->IsSeqString()),
-      chars_as_strings_(broker->zone()) {}
+      chars_as_strings_(broker->zone()) {
+  DCHECK(!FLAG_turbo_direct_heap_access);
+}
 
 class InternalizedStringData : public StringData {
  public: