[cfe] Add support for inline class field access

The representation field of an inline class is not part of the
generated code - it does not have a corresponding synthesized top-level
methods like the other inline class members. Therefore, to support
access to the representation field, a new `representationName` field
is added to `InlineClass` that holds the name by which the
representation field can be accessed.

An access to the representation field is at runtime an access of the
receiver itself. To show that the static type of the expression changes
from the inline type of the receiver to the declared representation
type, an `AsExpression` is inserted. Since this check is not needed
at runtime, this node is marked with the new `isUnchecked` flag, that
backends can use to skip the check.

TEST=pkg/front_end/testcases/inline_class/field_access.dart

Change-Id: I635af414af2ddc541352078766ce57a9d8ded601
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/277983
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>