Port IsBlittable fix and related changes from single-exe (#27746)

* Port set of changes from single-exe branch

* Enable precompilation of marshalling IL stubs in crossgen2 #26767
* P/invoke pregeneration fixes #27389
* Fixes to array marshalling pregeneration #27425
* Fix IsBlittableType #27436

* Fix stack overflow issue and m_alignpad==0 assert

I've found that the changes from single-exe had issues - it was causing
stack overflow during compilation of a couple of runtime assemblies.
After I've fixed that, there was about 15 tests failing at runtime failing
with assert failure: m_alignpad == 0. This is a well known indication of a
problem where JIT and crossgen get different field offsets, resulting in
writing beyond the end of an object.
The problem was caused by incorrect usage of sequential layout for class
without LayoutSequential attribute.
I've added a commit with fix for those to this PR.

src/tools/crossgen2/Common/TypeSystem/Ecma/EcmaField.cs

@@ -259,6 +259,20 @@ public override bool HasCustomAttribute(string attributeNamespace, string attrib
             return !MetadataReader.GetCustomAttributeHandle(MetadataReader.GetFieldDefinition(_handle).GetCustomAttributes(),
                 attributeNamespace, attributeName).IsNil;
         }
+
+        public override MarshalAsDescriptor GetMarshalAsDescriptor()
+        {
+            MetadataReader reader = MetadataReader;
+            FieldDefinition definition = reader.GetFieldDefinition(_handle);
+            if ((definition.Attributes & FieldAttributes.HasFieldMarshal) != 0)
+            {
+                BlobReader marshalAsReader = reader.GetBlobReader(definition.GetMarshallingDescriptor());
+                EcmaSignatureParser parser = new EcmaSignatureParser(_type.EcmaModule, marshalAsReader);
+                return parser.ParseMarshalAsDescriptor();
+            }
+
+            return null;
+        }
     }
 
     public static class EcmaFieldExtensions

src/tools/crossgen2/Common/TypeSystem/Ecma/EcmaSignatureParser.cs

@@ -397,7 +397,7 @@ public MarshalAsDescriptor ParseMarshalAsDescriptor()
             Debug.Assert(_reader.RemainingBytes != 0);
 
             NativeTypeKind type = (NativeTypeKind)_reader.ReadByte();
-            NativeTypeKind arraySubType = NativeTypeKind.Invalid;
+            NativeTypeKind arraySubType = NativeTypeKind.Default;
             uint? paramNum = null, numElem = null;
 
             switch (type)

src/tools/crossgen2/Common/TypeSystem/Ecma/EcmaType.cs

@@ -539,40 +539,6 @@ public override ClassLayoutMetadata GetClassLayout()
             return result;
         }
 
-        public override MarshalAsDescriptor[] GetFieldMarshalAsDescriptors()
-        {
-            var fieldDefinitionHandles = _typeDefinition.GetFields();
-
-            MarshalAsDescriptor[] marshalAsDescriptors = new MarshalAsDescriptor[fieldDefinitionHandles.Count];
-            int index = 0;
-            foreach (var handle in fieldDefinitionHandles)
-            {
-                var fieldDefinition = MetadataReader.GetFieldDefinition(handle);
-
-                if ((fieldDefinition.Attributes & FieldAttributes.Static) != 0)
-                    continue;
-
-                MarshalAsDescriptor marshalAsDescriptor = GetMarshalAsDescriptor(fieldDefinition);
-                marshalAsDescriptors[index++] = marshalAsDescriptor;
-            }
-
-            return marshalAsDescriptors;
-        }
-
-        private MarshalAsDescriptor GetMarshalAsDescriptor(FieldDefinition fieldDefinition)
-        {
-            if ((fieldDefinition.Attributes & FieldAttributes.HasFieldMarshal) == FieldAttributes.HasFieldMarshal)
-            {
-                MetadataReader metadataReader = MetadataReader;
-                BlobReader marshalAsReader = metadataReader.GetBlobReader(fieldDefinition.GetMarshallingDescriptor());
-                EcmaSignatureParser parser = new EcmaSignatureParser(EcmaModule, marshalAsReader);
-                MarshalAsDescriptor marshalAs =  parser.ParseMarshalAsDescriptor();
-                Debug.Assert(marshalAs != null);
-                return marshalAs;
-            }
-            return null;
-        }
-
         public override bool IsExplicitLayout
         {
             get

src/tools/crossgen2/Common/TypeSystem/Interop/FieldDesc.Interop.cs

@@ -0,0 +1,25 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace Internal.TypeSystem
+{
+    partial class FieldDesc
+    {
+        /// <summary>
+        /// Returns description of how the field should be marshalled to native code.
+        /// </summary>
+        public virtual MarshalAsDescriptor GetMarshalAsDescriptor()
+        {
+            return null;
+        }
+    }
+
+    partial class FieldForInstantiatedType
+    {
+        public override MarshalAsDescriptor GetMarshalAsDescriptor()
+        {
+            return _fieldDef.GetMarshalAsDescriptor();
+        }
+    }
+}