[cfe] Move lowering to constant evaluator

This moves the lowering of patterns in switch statement/expression,
pattern assignment, pattern variable declaration and if-case statement
to the constant evaluator.

Change-Id: Ic5810e96b26a74987c50fd71b306e41b59504e1f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/288401
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>

Author: johnniwinther

pkg/front_end/lib/src/fasta/kernel/constant_evaluator.dart

@@ -1668,6 +1668,7 @@ unsound
 unsubstituted
 untouched
 unusual
+unvisited
 unwrapper
 unwraps
 unwritten