[cfe] Remove use of Fragment in _Declaration

This removes the use of Fragment from _Declaration. Constructors and
properties are fully handled through their *Declaration objects. The
remaining declarations uses _StandardDeclaration which still uses
Fragment.

This is a step towards normalize builder creating between user
declarations and synthetic declarations.

Change-Id: Icb95ea42213248702f25a99e8c6f6b562d839bfa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/435821
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Chloe Stefantsova <[email protected]>

Author: johnniwinther