The ES2015 specification for switch statements 13.12.11 specifies that
they get their own lexical scope. This patch introduces such a scope
through a complex desugaring in terms of blocks, done so that Crankshaft
does not have to be updated to support multiple constructs providing
scopes.

BUG=v8:4377
LOG=Y
R=adamk

Review URL: https://codereview.chromium.org/1293283002

Cr-Commit-Position: refs/heads/master@{#30314}