Summary: We want to print all destructor stack-traces if Singleton destruction happens eventually (even after destroyInstances() was done).

Test Plan:
unit test

Reviewed By: chip@fb.com

Subscribers: folly-diffs@, yfeldblum

FB internal diff: D1929881

Signature: t1:1929881:1426878091:5993d544c0f84bdbf8bcdd420ef01fb7bf88ac3f