Handle promotion failures when scavenging a WEAK (#11108)

Previously, we ignored promotion failures when evacuating fields of
a WEAK object. When a failure happens, this resulted in an WEAK object
pointing to another object in a younger generation, causing crashes.

I used the test case from #11746 to check that the fix is working.
However I haven't managed to produce a test case that quickly reproduces
the issue.

Test Plan: ./validate

Reviewers: austin, bgamari, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2189

GHC Trac Issues: #11108

(cherry picked from commit 9363f04d)