Support JSDoc for type alias

src/compiler/parser.ts

@@ -5352,7 +5352,7 @@ namespace ts {
             parseExpected(SyntaxKind.EqualsToken);
             node.type = parseType();
             parseSemicolon();
-            return finishNode(node);
+            return addJSDocComment(finishNode(node));
         }
 
         // In an ambient declaration, the grammar only allows integer literals as initializers.

tests/cases/fourslash/jsDocForTypeAlias.ts

@@ -0,0 +1,7 @@
+///<reference path="fourslash.ts" />
+
+/////** DOC */
+////type /**/T = number
+
+goTo.marker();
+verify.quickInfoIs("type T = number", "DOC ");