Strip the ILLinkTrim.xml file from System.Private.Xml (#35547)

* Strip the ILLinkTrim.xml file from System.Private.Xml

The two methods being preserved in System.Private.Xml (CreateSqlReader and GenerateSerializer) are large, and are not necessary when linking a mobile app. These methods only need to be preserved when linking the shared framework. When a normal app is linked, the linker is able to figure out if the two methods are necessary.

Contributes to #35199

Fix #30912

* Respond to PR feedback

- Add PreserveDependencyAttribute to SqlXml to be explicit.
- Rename ILLinkTrim.xml to ILLinkTrim_LibraryBuild.xml when it shouldn't be embedded in the resulting assembly.

eng/illink.targets

@@ -29,6 +29,8 @@
     <ILLinkTrimOutputPath>$(IntermediateOutputPath)</ILLinkTrimOutputPath>
 
     <ILLinkTrimXml Condition="'$(ILLinkTrimXml)' == '' and Exists('$(MSBuildProjectDirectory)/ILLinkTrim.xml')">$(MSBuildProjectDirectory)/ILLinkTrim.xml</ILLinkTrimXml>
+    <!-- ILLinkTrim_LibraryBuild.xml files are only used during building the library, not an app. They shouldn't be embedded into the assembly. -->
+    <ILLinkTrimXmlLibraryBuild Condition="'$(ILLinkTrimXmlLibraryBuild)' == '' and Exists('$(MSBuildProjectDirectory)/ILLinkTrim_LibraryBuild.xml')">$(MSBuildProjectDirectory)/ILLinkTrim_LibraryBuild.xml</ILLinkTrimXmlLibraryBuild>
 
     <!-- if building a PDB, tell illink to rewrite the symbols file -->
     <ILLinkRewritePDBs Condition="'$(ILLinkRewritePDBs)' == '' and '$(DebugSymbols)' != 'false'">true</ILLinkRewritePDBs>
@@ -40,6 +42,10 @@
     </EmbeddedResource>
   </ItemGroup>
 
+  <ItemGroup Condition="'$(ILLinkTrimXmlLibraryBuild)' != ''">
+    <None Include="$(ILLinkTrimXmlLibraryBuild)" />
+  </ItemGroup>
+
   <!-- Custom binplacing for pre/post-trimming and reports that is useful for analysis 
        Must be enabled by setting BinPlaceILLinkTrimAssembly=true
   --> 
@@ -87,7 +93,9 @@
       <!-- keep types and members required by Debugger-related attributes -->
       <ILLinkArgs>$(ILLinkArgs) -v true</ILLinkArgs>
       <!-- don't remove the embedded root xml resource since ILLink may run again on the assembly -->
-      <ILLinkArgs>$(ILLinkArgs) --strip-descriptors false</ILLinkArgs>
+      <ILLinkArgs Condition="'$(ILLinkTrimXml)' != ''">$(ILLinkArgs) --strip-descriptors false</ILLinkArgs>
+      <!-- pass the non-embedded root xml file on the command line -->
+      <ILLinkArgs Condition="'$(ILLinkTrimXmlLibraryBuild)' != ''">$(ILLinkArgs) -x "$(ILLinkTrimXmlLibraryBuild)"</ILLinkArgs>
       <!-- ignore unresolved references -->
       <ILLinkArgs>$(ILLinkArgs) --skip-unresolved true</ILLinkArgs>
       <!-- keep interface implementations -->

src/libraries/System.Data.Common/src/System.Data.Common.csproj

@@ -294,6 +294,8 @@
     <Compile Include="System\Data\SQLTypes\SQLBytes.cs" />
     <Compile Include="System\Data\ProviderBase\DataReaderContainer.cs" />
     <Compile Include="System\Data\ProviderBase\SchemaMapping.cs" />
+    <Compile Include="$(CommonPath)System\Runtime\CompilerServices\PreserveDependencyAttribute.cs"
+             Link="Common\System\Runtime\CompilerServices\PreserveDependencyAttribute.cs" />
   </ItemGroup>
   <ItemGroup>
     <Reference Include="System.Collections" />

src/libraries/System.Data.Common/src/System/Data/SQLTypes/SqlXml.cs

@@ -10,6 +10,7 @@
 using System.Diagnostics;
 using System.Text;
 using System.Reflection;
+using System.Runtime.CompilerServices;
 
 namespace System.Data.SqlTypes
 {
@@ -127,6 +128,7 @@ private static Func<Stream, XmlReaderSettings, XmlParserContext, XmlReader> Crea
 
         private static MethodInfo CreateSqlReaderMethodInfo
         {
+            [PreserveDependency("CreateSqlReader", "System.Xml.XmlReader", "System.Private.Xml")]
             get
             {
                 if (s_createSqlReaderMethodInfo == null)