Put dummy types used in ProcessImportedType in the proper namespace

These dummy types were added in 0e66d19
so that the `ProcessImportedType` method could be added to the
`IDataContractSurrogate` interface properly. However, the types were not
put in the proper namespace.

Recent changes to the profile stubber to handle overloaded methods
correctly caused an additional `ProcessImportedType` method to be
incorrectly stubbed in. This change allows the profile stubber to
correctly notice that this method exists, and not stubbing is done.

Author: Josh Peterson

mcs/class/System/System.CodeDom/CodeCompileUnit.cs

@@ -1,7 +1,12 @@
 #if UNITY_AOT
 
+namespace System.CodeDom
+{
+
 public class CodeCompileUnit
 {
 }
 
+}
+
 #endif

mcs/class/System/System.CodeDom/CodeTypeDeclaration.cs

@@ -1,7 +1,12 @@
 #if UNITY_AOT
 
+namespace System.CodeDom
+{
+
 public class CodeTypeDeclaration
 {
 }
 
+}
+
 #endif