fix heap_vector_container::swap exception specification
fix heap_vector_container::swap exception specification

Summary:
* Using `this` is sometimes permitted but not always, and varies by compiler.
* Member `swap` is only expected to be available on lvalue references, not on prvalues or rvalue-references.

Fixes the oss build with gcc.

Reviewed By: Gownta

Differential Revision: D34345849

fbshipit-source-id: 95c7a9c15ea7f1c0316a0668754f3f76e61fce40

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