[cfe][augmentations] Support augmented declarations

This adds initial support for augmenting enum, mixin and extension type declarations, and unifies the implementation with the already existing support for classes and extensions.

Change-Id: I2f42a2471758d5cef398cff400d341a5982d3d9b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/543440
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>