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.

A class that is provided as Angular service is required to have an
`@Injectable()` decorator so that the compiler generates its injectable
definition for the runtime. Applications are automatically migrated
using the "missing-injectable" schematic, however libraries built for
older version of Angular may not yet satisfy this requirement.

This commit ports the "missing-injectable" schematic to a migration that
is ran when ngcc is processing a library. This ensures that any service
that is provided from an NgModule or Directive/Component will have an
`@Injectable()` decorator.

In ngcc's migration system, synthetic decorators can be injected into a
compilation to ensure that certain classes are compiled with Angular
logic, where the original library code did not include the necessary
decorators. Prior to this change, synthesized decorators would have a
fake AST structure as associated node and a made-up identifier. In
theory, this may introduce issues downstream:

1) a decorator's node is used for diagnostics, so it must have position
information. Having fake AST nodes without a position is therefore a
problem. Note that this is currently not a problem in practice, as
injected synthesized decorators would not produce any diagnostics.

2) the decorator's identifier should refer to an imported symbol.
Therefore, it is required that the symbol is actually imported.
Moreover, bundle formats such as UMD and CommonJS use namespaces for
imports, so a bare `ts.Identifier` would not be suitable to use as
identifier. This was also not a problem in practice, as the identifier
is only used in the `setClassMetadata` generated code, which is omitted
for synthetically injected decorators.

To remedy these potential issues, this commit makes a decorator's
identifier optional and switches its node over from a fake AST structure
to the class' name.

Previously, the (currently disabled) undecorated parent migration in
ngcc would produce errors when a base class could not be determined
statically or when a class extends from a class in another package. This
is not ideal, as it would cause the library to fail compilation without
a workaround, whereas those problems are not guaranteed to cause issues.

Additionally, inheritance chains were not handled. This commit reworks
the migration to address these limitations.

When upgrading an Angular application to a new version using the Angular
CLI, built-in schematics are being run to update user code from
deprecated patterns to the new way of working. For libraries that have
been built for older versions of Angular however, such schematics have
not been executed which means that deprecated code patterns may still be
present, potentially resulting in incorrect behavior.

Some of the logic of schematics has been ported over to ngcc migrations,
which are automatically run on libraries. These migrations achieve the
same goal of the regular schematics, but operating on published library
sources instead of used code.

This commit adds CopyDefinitionFeature, which supports the case where an
entire decorator (@component or @directive) is inherited from parent to
child.

The existing inheritance feature, InheritDefinitionFeature, supports merging
of parent and child definitions when both were originally present. This
merges things like inputs, outputs, host bindings, etc.

CopyDefinitionFeature, on the other hand, compensates for a definition that
was missing entirely on the child class, by copying fields that aren't
ordinarily inherited (like the template function itself).

This feature is intended to only be used as part of ngcc code generation.

In Angular View Engine, there are two kinds of decorator inheritance:

1) both the parent and child classes have decorators

This case is supported by InheritDefinitionFeature, which merges some fields
of the definitions (such as the inputs or queries).

2) only the parent class has a decorator

If the child class is missing a decorator, the compiler effectively behaves
as if the parent class' decorator is applied to the child class as well.
This is the "undecorated child" scenario, and this commit adds a migration
to ngcc to support this pattern in Ivy.

This migration has 2 phases. First, the NgModules of the application are
scanned for classes in 'declarations' which are missing decorators, but
whose base classes do have decorators. These classes are the undecorated
children. This scan is performed recursively, so even if a declared class
has a base class that itself inherits a decorator, this case is handled.

Next, a synthetic decorator (either @component or @directive) is created
on the child class. This decorator copies some critical information such
as 'selector' and 'exportAs', as well as supports any decorated fields
(@input, etc). A flag is passed to the decorator compiler which causes a
special feature `CopyDefinitionFeature` to be included on the compiled
definition. This feature copies at runtime the remaining aspects of the
parent definition which `InheritDefinitionFeature` does not handle,
completing the "full" inheritance of the child class' decorator from its
parent class.