Don't save changed parse options

The formatter wasn't undoing the change to add the DOTNET_FORMATTER
symbol to the parse options.  The Workspace code tried to persist this
to the project file and failed.  Remove it after formatting the files so
it isn't attempted to be persisted.

closes #73

Author: jaredpar

src/Microsoft.DotNet.CodeFormatting/FormattingEngineImplementation.cs

@@ -102,7 +102,7 @@ private async Task FormatAsync(Workspace workspace, IReadOnlyList<DocumentId> do
             FormatLogger.WriteLine("Total time {0}", watch.Elapsed);
         }
 
-        internal Solution AddTablePreprocessorSymbol(Solution solution)
+        private Solution AddTablePreprocessorSymbol(Solution solution)
         {
             var projectIds = solution.ProjectIds;
             foreach (var projectId in projectIds)
@@ -122,13 +122,32 @@ internal Solution AddTablePreprocessorSymbol(Solution solution)
             return solution;
         }
 
+        /// <summary>
+        /// Remove the added table preprocessor symbol.  Don't want that saved into the project
+        /// file as a change. 
+        /// </summary>
+        private Solution RemoveTablePreprocessorSymbol(Solution newSolution, Solution oldSolution)
+        {
+            var solution = newSolution;
+            var projectIds = solution.ProjectIds;
+            foreach (var projectId in projectIds)
+            {
+                var oldProject = oldSolution.GetProject(projectId);
+                var newProject = newSolution.GetProject(projectId);
+                solution = newProject.WithParseOptions(oldProject.ParseOptions).Solution;
+            }
+
+            return solution;
+        }
+
         internal async Task<Solution> FormatCoreAsync(Solution originalSolution, IReadOnlyList<DocumentId> documentIds, CancellationToken cancellationToken)
         {
             var solution = originalSolution;
             solution = AddTablePreprocessorSymbol(originalSolution);
             solution = await RunSyntaxPass(solution, documentIds, cancellationToken);
             solution = await RunLocalSemanticPass(solution, documentIds, cancellationToken);
             solution = await RunGlobalSemanticPass(solution, documentIds, cancellationToken);
+            solution = RemoveTablePreprocessorSymbol(solution, originalSolution);
             return solution;
         }