swift-api-digester: consider name alias type as a standalone node kind. (#5701)

* swift-api-digester: consider name alias type as a standalone node kind.

* [test] Update the swift-api-digester dump for stdlib to honor new name alias node.

Author: nkcsgexi

test/api-digester/Inputs/cake.swift

@@ -4,6 +4,7 @@ public struct S1 {
   internal func foo3() {}
   private func foo4() {}
   fileprivate func foo5() {}
+  public func foo6() -> Void {}
 }
 
 public class C1 {

test/api-digester/Outputs/cake.json

@@ -46,6 +46,29 @@
             }
           ]
         },
+        {
+          "kind": "Function",
+          "name": "foo6",
+          "printedName": "foo6()",
+          "declKind": "Func",
+          "usr": "s:FV4cake2S14foo6FT_T_",
+          "location": "",
+          "moduleName": "cake",
+          "children": [
+            {
+              "kind": "TypeNameAlias",
+              "name": "Void",
+              "printedName": "Void",
+              "children": [
+                {
+                  "kind": "TypeNominal",
+                  "name": "Void",
+                  "printedName": "()"
+                }
+              ]
+            }
+          ]
+        },
         {
           "kind": "Constructor",
           "name": "init",