Make XmlSerializer work with NativeAOT (#593)

Force the reflection-only code paths that don't Reflection.Emit.

Author: MichalStrehovsky

src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.targets

@@ -228,6 +228,9 @@ The .NET Foundation licenses this file to you under the MIT license.
       <IlcArg Include="@(_IlcRootedAssemblies->'--root:%(Identity)')" />
       <IlcArg Include="@(_IlcConditionallyRootedAssemblies->'--conditionalroot:%(Identity)')" />
 
+      <!-- The Emit-based XmlSerializer won't work -->
+      <IlcArg Include="--feature:System.Xml.Serialization.XmlSerializer.IsReflectionOnly=true" />
+
       <!-- Workaround for https://github.com/dotnet/corefx/issues/36723 -->
       <IlcArg Include="--removefeature:SerializationGuard" />
     </ItemGroup>

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

@@ -876,7 +876,7 @@ protected override void ProcessField(FieldDesc field)
 
             private object TryCreateSubstitution(TypeDesc type, string value)
             {
-                switch (type.Category)
+                switch (type.UnderlyingType.Category)
                 {
                     case TypeFlags.Int32:
                         if (string.IsNullOrEmpty(value))

src/libraries/System.Private.Xml/src/ILLink/ILLink.Substitutions.xml

@@ -0,0 +1,5 @@
+<linker>
+  <type fullname="System.Xml.Serialization.XmlSerializer" featurevalue="true" feature="System.Xml.Serialization.XmlSerializer.IsReflectionOnly">
+    <method signature="System.Xml.Serialization.SerializationMode get_Mode()" body="stub" value="1" />
+  </type>
+</linker>

src/libraries/System.Private.Xml/src/System.Private.Xml.csproj

@@ -5,6 +5,9 @@
     <TargetFrameworks>$(NetCoreAppCurrent)-windows;$(NetCoreAppCurrent)</TargetFrameworks>
     <Nullable>enable</Nullable>
   </PropertyGroup>
+  <ItemGroup>
+    <ILLinkSubstitutionsXmls Include="ILLink\ILLink.Substitutions.xml" />
+  </ItemGroup>
   <ItemGroup>
     <Compile Include="$(CommonPath)System\Text\StringBuilderCache.cs" Link="Common\System\StringBuilderCache.cs" />
     <Compile Include="$(CommonPath)System\HexConverter.cs" Link="Common\System\HexConverter.cs" />

src/libraries/System.Private.Xml/src/System/Xml/Serialization/XmlSerializer.cs

@@ -199,6 +199,12 @@ public XmlSerializer(Type type, string? defaultNamespace)
                 _primitiveType = type;
                 return;
             }
+
+            if (Mode == SerializationMode.ReflectionOnly)
+            {
+                return;
+            }
+
             _tempAssembly = s_cache[defaultNamespace, type];
             if (_tempAssembly == null)
             {