Add support for parsing and working with CustomModifiers in field signatures and make TypeSystemMetadataEmitter more flexible

src/coreclr/nativeaot/System.Private.TypeLoader/src/Internal/Runtime/TypeLoader/NativeLayoutFieldDesc.cs

@@ -39,6 +39,8 @@ public override TypeDesc FieldType
             }
         }
 
+        public override EmbeddedSignatureData[] GetEmbeddedSignatureData() => null;
+
         public override bool HasRva
         {
             get

src/coreclr/tools/Common/TypeSystem/Common/FieldDesc.cs

@@ -45,6 +45,9 @@ public abstract TypeDesc FieldType
             get;
         }
 
+        // Get the embedded signature data used to hold custom modifiers and such within a field signature
+        public abstract EmbeddedSignatureData[] GetEmbeddedSignatureData();
+
         public abstract bool IsStatic
         {
             get;

src/coreclr/tools/Common/TypeSystem/Common/FieldForInstantiatedType.cs

@@ -51,6 +51,11 @@ public override TypeDesc FieldType
             }
         }
 
+        public override EmbeddedSignatureData[] GetEmbeddedSignatureData()
+        {
+            return _fieldDef.GetEmbeddedSignatureData();
+        }
+
         public override bool IsStatic
         {
             get

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

@@ -113,6 +113,19 @@ public override TypeDesc FieldType
             }
         }
 
+        // This is extremely rarely needed. Don't cache it at all.
+        public override EmbeddedSignatureData[] GetEmbeddedSignatureData()
+        {
+            var metadataReader = MetadataReader;
+            BlobReader signatureReader = metadataReader.GetBlobReader(metadataReader.GetFieldDefinition(_handle).Signature);
+
+            EcmaSignatureParser parser = new EcmaSignatureParser(Module, signatureReader, NotFoundBehavior.Throw);
+            var fieldType = parser.ParseFieldSignature(out var embeddedSig);
+            Debug.Assert(fieldType == FieldType);
+            return embeddedSig;
+        }
+
+
         [MethodImpl(MethodImplOptions.NoInlining)]
         private int InitializeFieldFlags(int mask)
         {

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

@@ -471,6 +471,25 @@ public TypeDesc ParseFieldSignature()
             return ParseType();
         }
 
+        public TypeDesc ParseFieldSignature(out EmbeddedSignatureData[] embeddedSigData)
+        {
+            try
+            {
+                _indexStack = new Stack<int>();
+                _indexStack.Push(1);
+                _indexStack.Push(0);
+                _embeddedSignatureDataList = new List<EmbeddedSignatureData>();
+                TypeDesc parsedType = ParseFieldSignature();
+                embeddedSigData = _embeddedSignatureDataList.Count == 0 ? null : _embeddedSignatureDataList.ToArray();
+                return parsedType;
+            }
+            finally
+            {
+                _indexStack = null;
+                _embeddedSignatureDataList = null;
+            }
+        }
+
         public LocalVariableDefinition[] ParseLocalsSignature()
         {
             if (_reader.ReadSignatureHeader().Kind != SignatureKind.LocalVariables)

src/coreclr/tools/Common/TypeSystem/IL/Stubs/PInvokeLazyFixupField.cs

@@ -56,6 +56,8 @@ public override TypeDesc FieldType
             }
         }
 
+        public override EmbeddedSignatureData[] GetEmbeddedSignatureData() => null;
+
         public override bool HasRva
         {
             get

src/coreclr/tools/Common/TypeSystem/Interop/IL/InlineArrayType.cs

@@ -441,6 +441,7 @@ public override TypeDesc FieldType
                     return _owningType.ElementType;
                 }
             }
+            public override EmbeddedSignatureData[] GetEmbeddedSignatureData() => null;
 
             public override bool HasRva
             {

src/coreclr/tools/Common/TypeSystem/Interop/IL/NativeStructType.cs

@@ -348,6 +348,8 @@ public override TypeDesc FieldType
                 }
             }
 
+            public override EmbeddedSignatureData[] GetEmbeddedSignatureData() => null;
+
             public override bool HasRva
             {
                 get