Always raise a mem exceeded exception after a request OOM

Always raise a mem exceeded exception after a request OOM

Summary:
The introduction of host triggered request OOMs in D7230817 (https://github.com/facebook/hhvm/commit/3b613d1986fb1e73c221af2e4742e36474413263) repurposed the
MemExceededFlag.  It also made it so sometime when the MemExceededFlag is set,
we don't throw an exception.  This broke things that assume we always throw
an exception after a request level OOM.

This fix isn't the best option.  Ideally we would introduce an extra surprise
flag section so we don't have to manually multiplex the uses of the
MemExceededFlag.  For now this extends the hacks introduced in D7230817 (https://github.com/facebook/hhvm/commit/3b613d1986fb1e73c221af2e4742e36474413263).

Reviewed By: ricklavoie, binliu19

Differential Revision: D27886650

fbshipit-source-id: a6efe8e58331d8217836f10be83834a52fc63019