Apply workaround for linker issues

Author: captainsafia

eng/testing/linker/ILLink.Descriptors.xml

@@ -0,0 +1,5 @@
+<linker>
+  <assembly fullname="System.Private.CoreLib">
+    <type fullname="System.Array" />
+  </assembly>
+</linker>

eng/testing/linker/trimmingTests.targets

@@ -96,6 +96,8 @@
     <Copy SourceFiles="$(_projectSourceFile);
                        @(_additionalProjectSourceFiles)"
           DestinationFolder="$(_projectDir)" />
+    <Copy SourceFiles="$(MSBuildThisFileDirectory)ILLink.Descriptors.xml"
+          DestinationFolder="$(_projectDir)" />
     <Message Text="Generated $(_projectFile)" />
   </Target>