Elements. Centralize child fragment creation in ElementBuilder.

This commit refactors the process of associating child fragments with
their parent fragments within the summary2 element builder. The primary
goal is to reduce code duplication and improve maintainability by
centralizing the logic for creating these parent-child relationships.

Previously, the logic for adding a child fragment to its parent was
repeated in multiple locations throughout FragmentBuilder. This
typically involved:
   1. Retrieving the parent fragment from the current _enclosingContext.
   2. Manually setting the enclosingFragment property on the child.
   3. Calling _libraryBuilder.addFragmentChild to register the relationship.

These scattered and repetitive blocks of code have been replaced with a
single, unified helper method.

Key changes:

   - A new private method _addChildFragment(FragmentImpl child) is introduced in FragmentBuilder. This method encapsulates the entire process of linking a child fragment to its parent, which is retrieved from the current context.

   - The responsibility for setting the child.enclosingFragment property has been moved into LibraryBuilder.addChildFragment(). This ensures that the enclosing fragment is always set consistently whenever a child is added, making the process more robust.

   - All call sites within FragmentBuilder (for constructors, enum constants, fields, methods, etc.) have been updated to use the new _addChildFragment() method, significantly simplifying the code in visitor methods.

   - visitEnumDeclaration now wraps creating children fragments with `_withEnclosing` so that `_addChildFragment` can rely on having enclosing enum fragment.

   - As part of this refactoring, the point at which a ConstructorFragment is added to its enclosing InterfaceFragment was moved from FragmentBuilder.visitConstructorDeclaration to ElementBuilder.buildConstructorElement. This is a more logical location for this operation, as it occurs during the final element construction phase.

This refactoring does not change any analyzer behavior but improves the
internal code quality, making it cleaner, more readable, and easier to
maintain.

Change-Id: I254a1fbc23fe9f27e72ed7d15b0661d6cfa47816
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/441836
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

https://dart.googlesource.com/sdk/+/8acd5894e012d38b2ea3e58548a7f497c8e5dbf2