[strong] Allow mutually recursive classes.

The previous restrictions were overshooting (didn't allow a class to refer to a
later class under any circumstances); after this CL we're undershooting (allow
referring to any class from inside a method).

Implementing the correct checks (allow referring only if the class declarations
are in a consecutive block and if there's no dependency cycle) will be
implemented as a follow up.

BUG=v8:3956
LOG=N

Review URL: https://codereview.chromium.org/1087543004

Cr-Commit-Position: refs/heads/master@{#27888}