Migration: Rework FixAggregator to reduce order dependencies.

Previously, FixAggregator provided one NodeChange-derived class for
each possible kind of fix; if multiple fixes had to be made to a
single AST node, its client (FixBuilder) was required to nest them in
the appropriate order.  FixBuilder accomplished this by carefully
controlling the order in which it decided on fixes.  Unfortunately,
this careful control of order is becoming burdensome for FixBuilder to
maintain.

With this change, the class hierarchy of NodeChange mirrors the class
hierarchy of AstNode, with different fields for each possible kind of
fix.  This means that FixBuilder can decide on fixes in whatever order
is convenient.

Change-Id: I1c79cfc37b6289051b3927c67867b77a1dbb5ebf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/133541
Reviewed-by: Mike Fairhurst <mfairhurst@google.com>