fix(compiler): recognize @NgModule with a redundant @Injectable (#20320)

The compiler now, again, recognizes `@NgModule()` decorators on
classes with a redundant `@Injectable()` decorator.

Fixes: #19544

PR Close #20320

packages/compiler-cli/test/ngc_spec.ts

@@ -1392,6 +1392,42 @@ describe('ngc transformer command-line', () => {
   });
 
   describe('regressions', () => {
+    //#19544
+    it('should recognize @NgModule() directive with a redundant @Injectable()', () => {
+      write('src/tsconfig.json', `{
+        "extends": "../tsconfig-base.json",
+        "compilerOptions": {
+          "outDir": "../dist",
+          "rootDir": ".",
+          "rootDirs": [
+            ".",
+            "../dist"
+          ]
+        },
+        "files": ["test-module.ts"]
+      }`);
+      write('src/test.component.ts', `
+        import {Component} from '@angular/core';
+
+        @Component({
+          template: '<p>hello</p>',
+        })
+        export class TestComponent {}
+      `);
+      write('src/test-module.ts', `
+        import {Injectable, NgModule} from '@angular/core';
+        import {TestComponent} from './test.component';
+
+        @NgModule({declarations: [TestComponent]})
+        @Injectable()
+        export class TestModule {}
+      `);
+      const messages: string[] = [];
+      const exitCode =
+          main(['-p', path.join(basePath, 'src/tsconfig.json')], message => messages.push(message));
+      expect(exitCode).toBe(0, 'Compile failed unexpectedly.\n  ' + messages.join('\n  '));
+    });
+
     // #19765
     it('should not report an error when the resolved .css file is in outside rootDir', () => {
       write('src/tsconfig.json', `{