Add ProcessLinkerXmlBase to NativeAOT (#63666)

Add Xml Parsing linker files as a reference source to NativeAOT
Rename NativeAOT ProcessLinkerXmlBase version to ProcessXmlBase (uses XmlReader) 
Add ProcessLinkerXmlBase from linker and fix it so it can be used in NativeAOT (uses XPath)

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/FeatureSettings.cs

@@ -0,0 +1,52 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Xml.XPath;
+using ILLink.Shared;
+using Internal.TypeSystem;
+
+namespace ILCompiler
+{
+    public static class FeatureSettings
+    {
+        public static bool ShouldProcessElement(XPathNavigator nav, IReadOnlyDictionary<string, bool> featureSwitchValues)
+        {
+            var feature = GetAttribute(nav, "feature");
+            if (string.IsNullOrEmpty(feature))
+                return true;
+
+            var value = GetAttribute(nav, "featurevalue");
+            if (string.IsNullOrEmpty(value))
+            {
+                //context.LogError(null, DiagnosticId.XmlFeatureDoesNotSpecifyFeatureValue, documentLocation, feature);
+                return false;
+            }
+
+            if (!bool.TryParse(value, out bool bValue))
+            {
+                //context.LogError(null, DiagnosticId.XmlUnsupportedNonBooleanValueForFeature, documentLocation, feature);
+                return false;
+            }
+
+            var isDefault = GetAttribute(nav, "featuredefault");
+            bool bIsDefault = false;
+            if (!string.IsNullOrEmpty(isDefault) && (!bool.TryParse(isDefault, out bIsDefault) || !bIsDefault))
+            {
+                //context.LogError(null, DiagnosticId.XmlDocumentLocationHasInvalidFeatureDefault, documentLocation);
+                return false;
+            }
+
+            if (!featureSwitchValues.TryGetValue(feature, out bool featureSetting))
+                return bIsDefault;
+
+            return bValue == featureSetting;
+        }
+
+        public static string GetAttribute(XPathNavigator nav, string attribute)
+        {
+            return nav.GetAttribute(attribute, String.Empty);
+        }
+    }
+}

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/FeatureSwitchManager.cs

@@ -792,7 +792,7 @@ public MethodIL EmitIL(MethodDesc method)
             }
         }
 
-        private class SubstitutionsReader : ProcessLinkerXmlBase
+        private class SubstitutionsReader : ProcessXmlBase
         {
             private readonly Dictionary<MethodDesc, BodySubstitution> _methodSubstitutions;
             private readonly Dictionary<FieldDesc, object> _fieldSubstitutions;

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ManifestResourceBlockingPolicy.cs

@@ -100,7 +100,7 @@ public AssemblyFeatureInfo(EcmaModule module, IReadOnlyDictionary<string, bool>
             }
         }
 
-        private class SubstitutionsReader : ProcessLinkerXmlBase
+        private class SubstitutionsReader : ProcessXmlBase
         {
             private readonly HashSet<string> _substitutions = new HashSet<string>();