Async functions and generator declarations are only permitted as
StatementListItems, not as ExpressionStatements, and therefore not
as the entire body of an if statement, etc. Previously, they were
incorrectly permitted. However, ChakraCore and SpiderMonkey seem
to ban them in this context, and the feature was introduced relatively
recently, so it is likely to be web-compatible to ship the prohibition.

This patch also unifies the error message wording of async functions
and generators to ordinary functions, explaining more clearly what
the issue is.

Bug: v8:4483
Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I31ed7818d6ab3e7e325031bfabb933dbf4512143
Reviewed-on: https://chromium-review.googlesource.com/568979
Commit-Queue: Daniel Ehrenberg <littledan@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46770}