new small_vector tests that fail on trunk and uncover a bug in emplace_back on refere...

new small_vector tests that fail on trunk and uncover a bug in emplace_back on references to memory inside the vector + a fix + perf improvement to const lvalue push_back

Summary: emplace_back() on a small_vector applied on data inside the vector doesn't work properly.
In standard vectors, this usage is required to work properly, but I'm not sure whether it should in small_vector. Consider fixing / adding a lint rule.

Reviewed By: @yfeldblum

Differential Revision: D2122931