fix(language-service): prune duplicate returned definitions (#34995)

Sometimes, a request for definitions will return multiple of the same
definition. This can happen in at least the cases of

- two-way bindings (one of the same definition for the property and
  event binding)
- multiple template binding expressions in the same attribute
  - something like "*ngFor="let i of items; trackBy: test" has two
    template bindings, resulting in two template binding ASTs at the
    same location (the attribute span). The language service then parses
    both of these bindings individually, resulting in two independent
    but identical definitions. For more context, see https://github.com/angular/angular/pull/34847#discussion_r371006680.

This commit prunes duplicate definitions by signing definitions with
their location, and checking if that location signature has been seen in
a previous definition returned to the client.

PR Close #34995

packages/language-service/test/definitions_spec.ts

@@ -60,19 +60,16 @@ describe('definitions', () => {
     expect(textSpan).toEqual(marker);
     expect(definitions).toBeDefined();
 
-    // There are exactly two, indentical definitions here, corresponding to the "name" on the
-    // property and event bindings of the two-way binding. The two-way binding is effectively
-    // syntactic sugar for `[ngModel]="name" (ngModel)="name=$event"`.
-    expect(definitions !.length).toBe(2);
-    for (const def of definitions !) {
-      expect(def.fileName).toBe(PARSING_CASES);
-      expect(def.name).toBe('title');
-      expect(def.kind).toBe('property');
+    expect(definitions !.length).toBe(1);
+    const def = definitions ![0];
 
-      const fileContent = mockHost.readFile(def.fileName);
-      expect(fileContent !.substring(def.textSpan.start, def.textSpan.start + def.textSpan.length))
-          .toEqual(`title = 'Some title';`);
-    }
+    expect(def.fileName).toBe(PARSING_CASES);
+    expect(def.name).toBe('title');
+    expect(def.kind).toBe('property');
+
+    const fileContent = mockHost.readFile(def.fileName);
+    expect(fileContent !.substring(def.textSpan.start, def.textSpan.start + def.textSpan.length))
+        .toEqual(`title = 'Some title';`);
   });
 
   it('should be able to find a method from a call', () => {