Augment. Report augmentationOfDifferentDeclarationKind.

Keep track of the most recent same-named fragment even when it has a
different declaration kind. This lets augmentation validation
distinguish between a missing target and a target whose kind does not
match the augmentation.

Store this information on elements, serialize it through summaries, and
use it during error verification. When an augmentation finds a previous
declaration with the same name but a different kind, report
`augmentationOfDifferentDeclarationKind` and attach a context message
that points to the original declaration instead of falling back to
`augmentationWithoutDeclaration`.

This improves diagnostics for both top-level and member declarations,
including combinations such as constructors vs fields, methods vs
accessors, and functions vs variables.

Change-Id: Ic794c437fa160bb29d25f90559b544a7282ca697
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/494301
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>