Fix reporting GC fields from base types (dotnet#111011)

Fixes dotnet#110836.

When we extended managed CorInfoImpl to support object stack allocation in dotnet#104411, there was one more spot that assumed valuetypes only in `GatherClassGCLayout` that we missed. This resulted in not reporting any GC pointers in base types.

src/coreclr/inc/corinfo.h

@@ -2377,10 +2377,10 @@ class ICorStaticInfo
             bool                        fDoubleAlignHint = false
             ) = 0;
 
-    // This is only called for Value classes.  It returns a boolean array
-    // in representing of 'cls' from a GC perspective.  The class is
-    // assumed to be an array of machine words
-    // (of length // getClassSize(cls) / TARGET_POINTER_SIZE),
+    // Returns a boolean array representing 'cls' from a GC perspective.
+    // The class is assumed to be an array of machine words
+    // (of length getClassSize(cls) / TARGET_POINTER_SIZE for value classes
+    // and getHeapClassSize(cls) / TARGET_POINTER_SIZE for reference types),
     // 'gcPtrs' is a pointer to an array of uint8_ts of this length.
     // getClassGClayout fills in this array so that gcPtrs[i] is set
     // to one of the CorInfoGCType values which is the GC type of

src/coreclr/tools/Common/JitInterface/CorInfoImpl.cs

@@ -2312,6 +2312,10 @@ private int MarkGcField(byte* gcPtrs, CorInfoGCType gcType)
         private int GatherClassGCLayout(MetadataType type, byte* gcPtrs)
         {
             int result = 0;
+
+            if (type.MetadataBaseType is { ContainsGCPointers: true } baseType)
+                result += GatherClassGCLayout(baseType, gcPtrs);
+
             bool isInlineArray = type.IsInlineArray;
 
             foreach (var field in type.GetFields())