importTracker: Fix error for undefined importClause

src/services/importTracker.ts

@@ -212,6 +212,10 @@ namespace ts.FindAllReferences {
                 return;
             }
 
+            if (!decl.importClause) {
+                return;
+            }
+
             const { importClause } = decl;
 
             const { namedBindings } = importClause;

tests/cases/fourslash/findAllRefsNoImportClause.ts

@@ -0,0 +1,11 @@
+/// <reference path="fourslash.ts" />
+
+// https://github.com/Microsoft/TypeScript/issues/15452
+
+// @Filename: /a.ts
+////export const [|x|] = 0;
+
+// @Filename: /b.ts
+////import "./a";
+
+verify.singleReferenceGroup("const x: 0");