Add support for getting the output version

This was missed in an earlier PR.

src/Microsoft.CodeAnalysis.Razor.Workspaces/ProjectSystem/DefaultDocumentSnapshot.cs

@@ -60,6 +60,12 @@ namespace Microsoft.CodeAnalysis.Razor.ProjectSystem
             return output;
         }
 
+        public override async Task<VersionStamp> GetGeneratedOutputVersionAsync()
+        {
+            var (_, _, version) = await State.GetGeneratedOutputAndVersionAsync(ProjectInternal, this).ConfigureAwait(false);
+            return version;
+        }
+
         public override bool TryGetText(out SourceText result)
         {
             return State.TryGetText(out result);
@@ -81,5 +87,17 @@ namespace Microsoft.CodeAnalysis.Razor.ProjectSystem
             result = null;
             return false;
         }
+
+        public override bool TryGetGeneratedOutputVersionAsync(out VersionStamp result)
+        {
+            if (State.IsGeneratedOutputResultAvailable)
+            {
+                result = State.GetGeneratedOutputAndVersionAsync(ProjectInternal, this).Result.outputVersion;
+                return true;
+            }
+
+            result = default(VersionStamp);
+            return false;
+        }
     }
 }

src/Microsoft.CodeAnalysis.Razor.Workspaces/ProjectSystem/DefaultImportDocumentSnapshot.cs

@@ -37,6 +37,11 @@ namespace Microsoft.CodeAnalysis.Razor.ProjectSystem
             throw new NotSupportedException();
         }
 
+        public override Task<VersionStamp> GetGeneratedOutputVersionAsync()
+        {
+            throw new NotSupportedException();
+        }
+
         public override IReadOnlyList<DocumentSnapshot> GetImports()
         {
             return Array.Empty<DocumentSnapshot>();
@@ -81,5 +86,10 @@ namespace Microsoft.CodeAnalysis.Razor.ProjectSystem
         {
             throw new NotSupportedException();
         }
+
+        public override bool TryGetGeneratedOutputVersionAsync(out VersionStamp result)
+        {
+            throw new NotSupportedException();
+        }
     }
 }

src/Microsoft.CodeAnalysis.Razor.Workspaces/ProjectSystem/DocumentSnapshot.cs

@@ -26,10 +26,14 @@ namespace Microsoft.CodeAnalysis.Razor.ProjectSystem
 
         public abstract Task<RazorCodeDocument> GetGeneratedOutputAsync();
 
+        public abstract Task<VersionStamp> GetGeneratedOutputVersionAsync();
+
         public abstract bool TryGetText(out SourceText result);
 
         public abstract bool TryGetTextVersion(out VersionStamp result);
 
         public abstract bool TryGetGeneratedOutput(out RazorCodeDocument result);
+
+        public abstract bool TryGetGeneratedOutputVersionAsync(out VersionStamp result);
     }
 }