Add test for cref reference

src/EditorFeatures/CSharpTest/CodeActions/SyncNamespace/SyncNamespaceTests_ChangeNamespace.cs

@@ -160,9 +160,12 @@ namespace [||]{declaredNamespace}
     /// <summary>
     /// See <see cref=""Class1""/>
     /// See <see cref=""{declaredNamespace}.Class1""/>
+    /// See <see cref=""global::{declaredNamespace}.Class1""/>
+    /// See <see cref=""global::{declaredNamespace}.Class1.M1""/>
     /// </summary>
     public class Class1
     {{
+        public void M1() {{ }}
     }}
 }}</Document>
 <Document Folders=""{documentPath2.folder}"" FilePath=""{documentPath2.filePath}""> 
@@ -173,6 +176,8 @@ namespace Foo
     /// &lt;summary&gt;
     /// See &lt;see cref=""Class1""/&gt;
     /// See &lt;see cref=""{declaredNamespace}.Class1""/&gt;
+    /// See &lt;see cref=""global::{declaredNamespace}.Class1""/&gt;
+    /// See &lt;see cref=""global::{declaredNamespace}.Class1.M1""/&gt;
     /// &lt;/summary&gt;
     class RefClass
     {{
@@ -187,9 +192,12 @@ class RefClass
     /// <summary>
     /// See <see cref=""Class1""/>
     /// See <see cref=""Class1""/>
+    /// See <see cref=""global::A.B.C.Class1""/>
+    /// See <see cref=""global::A.B.C.Class1.M1""/>
     /// </summary>
     public class Class1
     {
+        public void M1() { }
     }
 }";
             var expectedSourceReference =
@@ -201,6 +209,8 @@ namespace Foo
     /// <summary>
     /// See <see cref=""Class1""/>
     /// See <see cref=""A.B.C.Class1""/>
+    /// See <see cref=""global::A.B.C.Class1""/>
+    /// See <see cref=""global::A.B.C.Class1.M1""/>
     /// </summary>
     class RefClass
     {

src/Features/CSharp/Portable/CodeRefactorings/SyncNamespace/CSharpSyncNamespaceService.cs

@@ -81,8 +81,6 @@ protected override SyntaxList<MemberDeclarationSyntax> GetMemberDeclarationsInCo
                 old = nameRef.Parent;
                 var aliasQualifier = GetAliasQualifierOpt(old);
 
-                // TODO: handle cref when changing namespace from global to non-global.
-
                 if (IsGlobalNamespace(newNamespaceParts))
                 {
                     // If new namespace is "", then name will be declared in global namespace.
@@ -103,7 +101,7 @@ protected override SyntaxList<MemberDeclarationSyntax> GetMemberDeclarationsInCo
 
             if (nameRef.Parent is NameMemberCrefSyntax crefName && crefName.Parent is QualifiedCrefSyntax qualifiedCref)
             {
-                // This is the case where the reference is a qualified name in `cref`.
+                // This is the case where the reference is the right most part of a qualified name in `cref`.
                 // for example, `<see cref="Foo.Baz.Bar"/>` and `<see cref="SomeAlias::Foo.Baz.Bar"/>`. 
                 // This is the form of `cref` we need to handle as a spacial case when changing namespace name or
                 // changing namespace from non-global to global, other cases in these 2 scenarios can be handled in the