[cfe] Refactor _FragmentName

This refactors the _FragmentName class to a more encapsulated approach
in preparation for creating synthetic declarations together with the
user defined declarations.

_FragmentName is renamed to _Declaration and a subhierarchy is created
to handle the various kinds of declarations and let them control how
the corresponding _PreBuilder objects are created.

Change-Id: I5ba3f238112caeb28f8ce6938f15905e512cab6f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/432620
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Chloe Stefantsova <[email protected]>

Author: johnniwinther

pkg/front_end/lib/src/fragment/enum_element.dart

@@ -104,7 +104,6 @@ class EnumElementDeclaration
   TypeBuilder get type => _fragment.type;
 
   @override
-  // Coverage-ignore(suite): Not run.
   UriOffsetLength get uriOffset => _fragment.uriOffset;
 
   @override

pkg/front_end/lib/src/fragment/primary_constructor_field.dart

@@ -98,7 +98,6 @@ class PrimaryConstructorFieldDeclaration
   TypeBuilder get type => _fragment.type;
 
   @override
-  // Coverage-ignore(suite): Not run.
   UriOffsetLength get uriOffset => _fragment.uriOffset;
 
   // Coverage-ignore(suite): Not run.