Clusterfuzz testing discovered that sloppy-mode block-scoped function
declarations introduce lexically-scoped variables in scopes that were
thrown away under the expectation that no lexically-scoped variables
were introduced. These cases are:

  for (;;) function foo() {}
  for (x in y) function foo() {}

This patch ensures that a block is created in those cases to hold the
lexically scoped variable. Usually, scope analysis should discover that
that block is not important, and it should not have a runtime
representation.

BUG=chromium:536750,chromium:536751
LOG=Y
R=adamk

Review URL: https://codereview.chromium.org/1382123002

Cr-Commit-Position: refs/heads/master@{#31109}