folly::Singleton leak diagnostics utils

folly::Singleton leak diagnostics utils

Summary: This adds different types of messages depending on whether Singleton was depending on other Singleton or just leaked. It also adds destruction stack trace for such Singletons (if they were ever destroyed) to help debug such leaks/broken dependencies.

Test Plan: unit test

Reviewed By: chip@fb.com

Subscribers: trunkagent, folly-diffs@, yfeldblum

FB internal diff: D1830526

Signature: t1:1830526:1423266462:ba328b0da0bf4030b1c4f686d8f7b609fd20683c