SingeltonVault::ScopedExpunger - RAII class to clear singletons

SingeltonVault::ScopedExpunger - RAII class to clear singletons

Summary:
[Folly] `SingeltonVault::ScopedExpunger` - RAII class to clear singletons.

Clears all singletons in the given vault at ctor and dtor times. Useful for unit-tests that need to clear the world.

This need can arise when a unit-test needs to swap out an object used by a singleton for a test-double, but the singleton needing its dependency to be swapped has a type or a tag local to some other translation unit and unavailable in the current translation unit.

Other, better approaches to this need are "plz 2 refactor" ....

Reviewed By: andriigrynenko

Differential Revision: D2802459

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