F14Table: Specialize reserve() for default-constructed objects
F14Table: Specialize reserve() for default-constructed objects

Summary: During construction of F14Table with a specific capacity, we call `reserve()`. `reserve()` is a general function that has to work with an already-constructed map. We can go faster during construction by creating a specialized code path.

Reviewed By: ot

Differential Revision: D39362045

fbshipit-source-id: b45b9ac805a871389a46b81bb509c2e17b3a827a

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly