[analyzer] Produce CONST_EVAL_EXTENSION_METHOD diagnostic when using extension methods in const contexts.

More specific error code.

Semi-related to work in https://dart-review.googlesource.com/c/sdk/+/301505.

Change-Id: I1233e9e31389d387a9c777fe83cdd30a224bd00e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/310760
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>

https://dart.googlesource.com/sdk/+/9ec1a84f2939a46e9976164017d0159b823120db