This is another step towards lazily allocating them in the block state.
ClassLiteral should also have a lazy block-scope for the outermost scope,
but currently that doesn't work due to the parameter initializer rewriter
and minor implementation details in ignition and turbofan.

BUG=v8:5209

Review-Url: https://codereview.chromium.org/2166843003
Cr-Commit-Position: refs/heads/master@{#38196}