[analyzer,cfe] Add support for macro class

This CL add the 'macros' experiment and adds support for the `macro`
modifier on class declarations. This is part of the prototyping of
the static meta-programming feature.

Change-Id: Ie4b8c5443fda3813307ea076c315baad6a90c3e9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/224205
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

tools/experimental_features.yaml

@@ -131,6 +131,9 @@ features:
   super-parameters:
     help: "Super-Initializer Parameters"
 
+  macros:
+    help: "Static meta-programming"
+
 # Experiment flag only used for testing.
   test-experiment:
     help: >-