Skip analyzing files with <autogenerated /> in its header

We already recognized <auto-generated />. The original StyleCop also recognized <autogenerated />. So this brings us up to parity with that.

Fixes DotNetAnalyzers#833

StyleCop.Analyzers/StyleCop.Analyzers.Test/DocumentationRules/SA1600UnitTests.cs

@@ -915,6 +915,9 @@ public void SomeMethod() { }
             await this.VerifyCSharpDiagnosticAsync(testCodeWithDocumentation, EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
         }
 
+        /// <summary>
+        /// Verifies that we recognize the auto-generated xml element.
+        /// </summary>
         [Fact]
         public async Task TestSkipAutoGeneratedCodeAsync()
         {
@@ -928,6 +931,30 @@ public async Task TestSkipAutoGeneratedCodeAsync()
 // </auto-generated>
 //------------------------------------------------------------------------------
 
+public class OuterClass
+{
+    public void SomeMethod() { }
+}";
+
+            await this.VerifyCSharpDiagnosticAsync(testCode, EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+        }
+
+        /// <summary>
+        /// Verifies that we recognize the autogenerated xml element.
+        /// </summary>
+        [Fact]
+        public async Task TestSkipAutoGeneratedCode2Async()
+        {
+            var testCode = @"//------------------------------------------------------------------------------
+// <autogenerated>
+//     This code was generated by a tool.
+//     Runtime Version:4.0.30319.0
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </autogenerated>
+//------------------------------------------------------------------------------
+
 public class OuterClass
 {
     public void SomeMethod() { }

StyleCop.Analyzers/StyleCop.Analyzers/GeneratedCodeAnalysisExtensions.cs

@@ -164,7 +164,10 @@ private static bool HasAutoGeneratedComment(SyntaxTree tree, CancellationToken c
             }
 
             var comments = trivia.Where(t => t.IsKind(SyntaxKind.SingleLineCommentTrivia) || t.IsKind(SyntaxKind.MultiLineCommentTrivia));
-            return comments.Any(t => t.ToString().Contains("<auto-generated"));
+            return comments.Any(t => {
+                string s = t.ToString();
+                return s.Contains("<auto-generated") || s.Contains("<autogenerated");
+            });
         }
 
         /// <summary>