Implements https://github.com/tc39/ecma262/issues/2034

Currently the token sequence `for (async of` is ambiguous. It can be the
prefix for either `(async of => {};;);` or `for (async of foo);`. This
CL disallows the token sequence.

Note that `for await (async of` is still allowed, since there is no
C-style `for await (;;)`, and thus no ambiguity.

Bug: v8:11412
Change-Id: I3fede83a69420996baa2bc8b6c1cff000535d990
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2683221
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72607}