feat(core): properly support inheritance

## Inheritance Semantics:

Decorators:
1) list the decorators of the class and its parents in the ancestor first order
2) only use the last decorator of each kind (e.g. @Component / ...)

Constructor parameters:
If a class inherits from a parent class and does not declare
a constructor, it inherits the parent class constructor,
and with it the parameter metadata of that parent class.

Lifecycle hooks:
Follow the normal class inheritance model,
i.e. lifecycle hooks of parent classes will be called
even if the method is not overwritten in the child class.

## Example

E.g. the following is a valid use of inheritance and it will
also inherit all metadata:

```
@Directive({selector: 'someDir'})
class ParentDirective {
  constructor(someDep: SomeDep) {}

  ngOnInit() {}
}

class ChildDirective extends ParentDirective {}
```

Closes #11606
Closes #12892

modules/@angular/compiler-cli/src/compiler_host.ts

@@ -186,7 +186,7 @@ export class CompilerHost implements AotCompilerHost {
       if (!v2Metadata && v1Metadata) {
         // patch up v1 to v2 by merging the metadata with metadata collected from the d.ts file
         // as the only difference between the versions is whether all exports are contained in
-        // the metadata
+        // the metadata and the `extends` clause.
         v2Metadata = {'__symbolic': 'module', 'version': 2, 'metadata': {}};
         if (v1Metadata.exports) {
           v2Metadata.exports = v1Metadata.exports;

modules/@angular/compiler-cli/test/aot_host_spec.ts

@@ -163,7 +163,11 @@ describe('CompilerHost', () => {
       {__symbolic: 'module', version: 1, metadata: {foo: {__symbolic: 'class'}}}, {
         __symbolic: 'module',
         version: 2,
-        metadata: {foo: {__symbolic: 'class'}, bar: {__symbolic: 'class'}}
+        metadata: {
+          foo: {__symbolic: 'class'},
+          Bar: {__symbolic: 'class', members: {ngOnInit: [{__symbolic: 'method'}]}},
+          BarChild: {__symbolic: 'class', extends: {__symbolic: 'reference', name: 'Bar'}}
+        }
       }
     ]);
   });
@@ -198,7 +202,12 @@ const FILES: Entry = {
         }
       },
       'metadata_versions': {
-        'v1.d.ts': 'export declare class bar {}',
+        'v1.d.ts': `
+          export declare class Bar {
+            ngOnInit() {}
+          }
+          export declare class BarChild extends Bar {}
+        `,
         'v1.metadata.json':
             `{"__symbolic":"module", "version": 1, "metadata": {"foo": {"__symbolic": "class"}}}`,
       }