Fix exception_wrapper on Windows by abusing internals

Fix exception_wrapper on Windows by abusing internals

Summary:
exception_wrapper's non-allocating variant was depending
on the exception reference being a reference to a heap object,
which is not the case on Windows. All thrown exceptions are
copied to the stack first.

So, we abuse internals of exception_ptr to get the internally
heap allocated copy of the object. What could possibly go wrong.

Reviewed By: yfeldblum, ericniebler, Orvid

Differential Revision: D5270634

fbshipit-source-id: 2983a6af9a2c3cb66cbee8a382707a76b582c489