[cfe/ffi] Fix user-defined getters

https://dart-review.googlesource.com/c/sdk/+/198281 introduced support
for incremental compilation to the FfiTransform. This included reading
fields from already transformed structs and unions.

However, fields which already had been transformed are indistinguishable
from user-defined getters.

So instead of re-reading the information from the fields, for already
transformed structs, we read the information from the
`vm:ffi:struct-fields` pragma on the struct class.

Closes: https://github.com/dart-lang/sdk/issues/46004

TEST=tests/ffi/regress_46004_test.dart
TEST=pkg/front_end/testcases/incremental/regress_46004.yaml

Change-Id: Iebffda037913e71349bba9685dc16e2f478a0e7b
Cq-Include-Trybots: luci.dart.try:vm-kernel-win-debug-x64-try,front-end-nnbd-linux-release-x64-try,front-end-linux-release-x64-try,vm-ffi-android-debug-arm64-try,vm-precomp-ffi-qemu-linux-release-arm-try
Fixed: 46004
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/200640
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Clement Skau <cskau@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>