Handle rename decl conflicts when decl file contains no refs

Fixes #2352

This change fixes a rename crash that would occur when a method
declaration conflict was introduced in a file with no references to that
symbol, but where at least one other document in the same project had a
reference to that symbol.

When processing an individual project in ResolveConflictsAsync, we start
with the full set of documents from _documentsIdsToBeCheckedForConflict
related to that project. But, each rename phase can adjust this set of
documents (called documentIdsThatGetsAnnotatedAndRenamed locally) to
whatever documents are represented by the current set of
_conflictLocations, which can exclude the
_documentIdOfRenameSymbolDeclaration itself if it contains no
references. Now, when documentIdsThatGetsAnnotatedAndRenamed is
recalculated we make sure to keep the
_documentIdOfRenameSymbolDeclaration in the set if we're processing the
project that contains it.

src/EditorFeatures/Test2/Rename/RenameEngineTests.CSharpConflicts.vb

@@ -3344,5 +3344,72 @@ class {|conflict:C$$|} { }
                 result.AssertLabeledSpansAre("conflict", renameTo, RelatedLocationType.UnresolvedConflict)
             End Using
         End Sub
+
+        <WorkItem(2352, "https://github.com/dotnet/roslyn/issues/2352")>
+        <Fact>
+        <Trait(Traits.Feature, Traits.Features.Rename)>
+        Public Sub DeclarationConflictInFileWithoutReferences_SameProject()
+            Using result = RenameEngineResult.Create(
+                   <Workspace>
+                       <Project Language="C#" CommonReferences="true">
+                           <Document FilePath="Test1.cs">
+class Program
+{
+    internal void [|A$$|]() { }
+    internal void {|conflict:B|}() { }
+}
+                            </Document>
+                           <Document FilePath="Test2.cs">
+class Program2
+{
+    void M()
+    {
+        Program p = null;
+        p.{|conflict:A|}();
+        p.{|conflict:B|}();
+    }
+}
+                            </Document>
+                       </Project>
+                   </Workspace>, renameTo:="B")
+
+                result.AssertLabeledSpansAre("conflict", "B", RelatedLocationType.UnresolvedConflict)
+            End Using
+        End Sub
+
+        <WorkItem(2352, "https://github.com/dotnet/roslyn/issues/2352")>
+        <Fact>
+        <Trait(Traits.Feature, Traits.Features.Rename)>
+        Public Sub DeclarationConflictInFileWithoutReferences_DifferentProjects()
+            Using result = RenameEngineResult.Create(
+                   <Workspace>
+                       <Project Language="C#" CommonReferences="true" AssemblyName="CSAssembly1">
+                           <Document FilePath="Test1.cs">
+public class Program
+{
+    public void [|A$$|]() { }
+    public void {|conflict:B|}() { }
+}
+                            </Document>
+                       </Project>
+                       <Project Language="C#" CommonReferences="true" AssemblyName="CSAssembly2">
+                           <ProjectReference>CSAssembly1</ProjectReference>
+                           <Document FilePath="Test2.cs">
+class Program2
+{
+    void M()
+    {
+        Program p = null;
+        p.{|conflict:A|}();
+        p.{|conflict:B|}();
+    }
+}
+                            </Document>
+                       </Project>
+                   </Workspace>, renameTo:="B")
+
+                result.AssertLabeledSpansAre("conflict", "B", RelatedLocationType.UnresolvedConflict)
+            End Using
+        End Sub
     End Class
 End Namespace

src/Workspaces/Core/Portable/Rename/ConflictEngine/ConflictResolver.Session.cs

@@ -170,6 +170,14 @@ public async Task<ConflictResolution> ResolveConflictsAsync()
                             // Note that we need to get the conflictLocations here since we're going to remove some locations below if phase == 2
                             documentIdsThatGetsAnnotatedAndRenamed = new HashSet<DocumentId>(_conflictLocations.Select(l => l.DocumentId));
 
+                            // Include the declaration document if we are processing its project to
+                            // ensure declaration conflicts are calculated correctly when the
+                            // declaration document does not contain any references.
+                            if (documentsByProject.Key == _documentIdOfRenameSymbolDeclaration.ProjectId)
+                            {
+                                documentIdsThatGetsAnnotatedAndRenamed.Add(_documentIdOfRenameSymbolDeclaration);
+                            }
+
                             if (phase == 2)
                             {
                                 // After phase 2, if there are still conflicts then remove the conflict locations from being expanded