fix(ngcc): prevent reflected decorators from being clobbered (#33362)

ngcc has an internal cache of computed decorator information for
reflected classes, which could previously be mutated by consumers of the
reflection host. With the ability to inject synthesized decorators, such
decorators would inadvertently be added into the array of decorators
that was owned by the internal cache of the reflection host, incorrectly
resulting in synthesized decorators to be considered real decorators on
a class. This commit fixes the issue by cloning the cached array before
returning it.

PR Close #33362

packages/compiler-cli/ngcc/test/host/esm2015_host_spec.ts

@@ -1931,6 +1931,20 @@ runInEachFileSystem(() => {
         expect(classDecoratorsSecondary.length).toEqual(1);
         expect(classDecoratorsSecondary[0] !.map(d => d.name)).toEqual(['Directive']);
       });
+
+      it('should return a cloned array on each invocation', () => {
+        loadTestFiles(DECORATED_FILES);
+        const {program} = makeTestBundleProgram(getRootFiles(DECORATED_FILES)[0]);
+        const host = new Esm2015ReflectionHost(new MockLogger(), false, program.getTypeChecker());
+        const classDecl =
+            getDeclaration(program, DECORATED_FILES[0].name, 'A', ts.isClassDeclaration) !;
+        const classSymbol = host.getClassSymbol(classDecl) !;
+
+        const firstResult = host.getDecoratorsOfSymbol(classSymbol);
+        const secondResult = host.getDecoratorsOfSymbol(classSymbol);
+
+        expect(firstResult).not.toBe(secondResult);
+      });
     });
 
     describe('getDtsDeclarationsOfClass()', () => {