.NET Core WPF Build error on custom BaseIntermediateOutputPath #1718 (#3120)

* Use more robust relative path calculation in markup code generation

Author: ryalanms

src/Microsoft.DotNet.Wpf/src/PresentationBuildTasks/MS/Internal/MarkupCompiler/MarkupCompiler.cs

@@ -1575,19 +1575,11 @@ private string ParentFolderPrefix
         {
             get
             {
-                string parentFolderPrefix = string.Empty;
-                if (TargetPath.StartsWith(SourceFileInfo.SourcePath, StringComparison.OrdinalIgnoreCase))
-                {
-                    string relPath = TargetPath.Substring(SourceFileInfo.SourcePath.Length);
-                    relPath += SourceFileInfo.RelativeSourceFilePath;
-                    string[] dirs = relPath.Split(new Char[] { Path.DirectorySeparatorChar });
-                    for (int i = 1; i < dirs.Length; i++)
-                    {
-                        parentFolderPrefix += PARENTFOLDER;
-                    }
-                }
-
-                return parentFolderPrefix;
+#if NETFX 
+                return PathInternal.GetRelativePath(TargetPath, SourceFileInfo.SourcePath, StringComparison.OrdinalIgnoreCase);
+#else
+                return Path.GetRelativePath(TargetPath, SourceFileInfo.SourcePath);
+#endif
             }
         }