[postmortem] update Symbol and *String metadata

Symbol and *String classes are now declared on Torque with
generateCppClass, which means they don't use macro accessors anymore. As
such, the gen-postmortem-metadata script is not able to automatically
detect fields for those classes. Define metadata for those fields
manually for now. In the future we might want to generate it from Torque
for consistency.

Also renamed a few *String fields metadata to match the expected format
(className__fieldName__fieldType). For more context:
https://github.com/nodejs/llnode/issues/287#issuecomment-539707117.

R=bmeurer@chromium.org, hpayer@chromium.org, verwaest@chromium.org, yangguo@chromium.org

Change-Id: I82fe8315cdbfd1b8c64c6a8d5dc011b1edaec39e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1847783
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64313}

tools/gen-postmortem-metadata.py

@@ -229,15 +229,6 @@ consts_misc = [
 
     { 'name': 'class_SharedFunctionInfo__function_data__Object',
         'value': 'SharedFunctionInfo::kFunctionDataOffset' },
-
-    { 'name': 'class_ConsString__first_offset__int',
-        'value': 'ConsString::kFirstOffset' },
-    { 'name': 'class_ConsString__second_offset__int',
-        'value': 'ConsString::kSecondOffset' },
-    { 'name': 'class_SlicedString__offset_offset__int',
-        'value': 'SlicedString::kOffsetOffset' },
-    { 'name': 'class_ThinString__actual_offset__int',
-        'value': 'ThinString::kActualOffset' },
 ];
 
 #
@@ -286,6 +277,11 @@ extras_accessors = [
     'Code, instruction_size, int, kInstructionSizeOffset',
     'String, length, int32_t, kLengthOffset',
     'DescriptorArray, header_size, uintptr_t, kHeaderSize',
+    'ConsString, first, String, kFirstOffset',
+    'ConsString, second, String, kSecondOffset',
+    'SlicedString, offset, SMI, kOffsetOffset',
+    'ThinString, actual, String, kActualOffset',
+    'Symbol, name, Object, kNameOffset',
 ];
 
 #