For generator-based functions (e.g. async functions) we force variables to be
context-allocated.  Due to a bug in the parser, this didn't always work
correctly.  For instance, in "async function foo([a]) { ... }" the variable "a"
could become stack-allocated due to context allocation being forced on the wrong
scope.

Besides fixing this, I'm also cleaning up some related code in the async parsing
setup and adding some guards.

R=adamk@chromium.org, littledan@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2561093002
Cr-Commit-Position: refs/heads/master@{#41635}