Elements. Perform mixin inference on InterfaceElementImpl, not on

Previously, mixin inference data was collected for each interface
fragment individually, including augmentations. This data was then
combined, which required managing separate information for the
declaration and each augmentation, as well as handling substitutions
between them.

This change refactors the mixin inference process to be associated with
the whole `InterfaceElementImpl` rather than individual fragments.

The `TypesBuilder` now collects all fragments that have a `with` clause
for a given element. A single `_MixinsInference` instance then iterates
through these fragments and performs inference for each one within the
context of the whole element. This is orchestrated by the new
`_addFragmentWithClause` method, which gathers all relevant fragments
(both declarations and augmentations) under their parent element.

This new approach simplifies the logic significantly:
- It removes the need for the previous complex logic that handled augmentations separately in `_inferDeclaration`. Augmentations are now treated uniformly with the base declaration.
- It allows for the removal of redundant calls to the old `_updatedAugmented` function for declarations like `extension` and `mixin` that do not support mixins themselves.

This refactoring makes the mixin inference logic cleaner and easier to
follow, especially with the presence of augmentations, while preserving
the existing functionality.

The functionality is the same.

Change-Id: Iaef78af37e76eeb82e2600112fcc2b483cce5bad
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/442284
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>