[lldb/Dataformatters] Adapt C++ std::string dataformatter for D128285

This patch changes the C++ `std::string` dataformatter to reflect
internal layout changes following D128285.

Now, in short-mode strings, in order to access the `__size_` and
`__is_long_` attributes, we need to access a packed anonymous struct,
which introduces another indirection.

We need to do the same in order to access the `__cap_` field for
long-mode strings.

This should fix the various test failures that are happening on
GreenDragon:

https://green.lab.llvm.org/green/job/lldb-cmake/44918/

rdar://96010248

Differential Revision: https://reviews.llvm.org/D128694

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>

lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp

@@ -583,8 +583,18 @@ ExtractLibcxxStringInfo(ValueObject &valobj) {
                                       : eLibcxxStringLayoutModeCSD;
   uint64_t size_mode_value = 0;
 
-  if (ValueObjectSP is_long = dataval_sp->GetChildAtNamePath(
-          {ConstString("__s"), ConstString("__is_long_")})) {
+  ValueObjectSP short_sp(dataval_sp->GetChildAtIndex(1, true));
+  if (!short_sp)
+    return {};
+
+  // After D128285, we need to access the `__is_long_` and `__size_` fields from
+  // a packed anonymous struct
+  ValueObjectSP packed_fields_sp = short_sp->GetChildAtIndex(0, true);
+  if (!packed_fields_sp)
+    return {};
+
+  if (ValueObjectSP is_long = packed_fields_sp->GetChildMemberWithName(
+          ConstString("__is_long_"), true)) {
     using_bitmasks = false;
     short_mode = !is_long->GetValueAsUnsigned(/*fail_value=*/0);
     if (ValueObjectSP size_member =
@@ -621,14 +631,11 @@ ExtractLibcxxStringInfo(ValueObject &valobj) {
   }
 
   if (short_mode) {
-    ValueObjectSP short_sp(dataval_sp->GetChildAtIndex(1, true));
-    if (!short_sp)
-      return {};
-    ValueObjectSP location_sp = short_sp->GetChildAtIndex(
+    ValueObjectSP location_sp = packed_fields_sp->GetChildAtIndex(
         (layout == eLibcxxStringLayoutModeDSC) ? 0 : 1, true);
     // After D125496, there is a flat layout.
     if (location_sp->GetName() == g_size_name)
-      location_sp = short_sp->GetChildAtIndex(3, true);
+      location_sp = short_sp->GetChildMemberWithName(g_data_name, true);
     if (using_bitmasks)
       size = (layout == eLibcxxStringLayoutModeDSC)
                  ? size_mode_value
@@ -650,13 +657,19 @@ ExtractLibcxxStringInfo(ValueObject &valobj) {
   ValueObjectSP l(dataval_sp->GetChildAtIndex(0, true));
   if (!l)
     return {};
+
+  // After D128285, we need to access the `__cap_` field from a packed anonymous
+  // struct
+  packed_fields_sp = l->GetChildAtIndex(0, true);
+  if (!packed_fields_sp)
+    return {};
   // we can use the layout_decider object as the data pointer
   ValueObjectSP location_sp =
       l->GetChildMemberWithName(ConstString("__data_"), /*can_create=*/true);
   ValueObjectSP size_vo =
       l->GetChildMemberWithName(ConstString("__size_"), /*can_create=*/true);
-  ValueObjectSP capacity_vo =
-      l->GetChildMemberWithName(ConstString("__cap_"), /*can_create=*/true);
+  ValueObjectSP capacity_vo = packed_fields_sp->GetChildMemberWithName(
+      ConstString("__cap_"), /*can_create=*/true);
   if (!size_vo || !location_sp || !capacity_vo)
     return {};
   size = size_vo->GetValueAsUnsigned(LLDB_INVALID_OFFSET);