If an inner function only declares a variable but doesn't use it, Parser
and PreParser produced different unresolved variables, and that confused
the pessimistic context allocation.

This is continuation to https://codereview.chromium.org/2388183003/

This CL fixes more complicated declarations (which are not just one
identifier). For this, PreParser needs to accumulate identifiers used
in expressions.

In addition, this CL manifests FLAG_lazy_inner_functions in tests, so that
we get clusterfuzz coverage for it.

BUG=chromium:650969, v8:5501

Review-Url: https://codereview.chromium.org/2400613003
Cr-Commit-Position: refs/heads/master@{#40112}