[cfe] Add FactoryDeclaration

This adds a FactoryDeclaration to separate the SourceFactoryBuilder from the fragment and encoding.

This is done in preparation for handling factory patches through fragments.

Change-Id: Ibb504b46ef33ac348053f0cabafcddf75fc580b8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/415560
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Jens Johansen <[email protected]>

Author: johnniwinther