Summary: [Folly] Helper variable templates for tuple applicability traits: `is_applicable_v`, `is_applicable_r_v`, `is_nothrow_applicable_v`, `is_nothrow_applicable_r_v`.

Reviewed By: lewissbaker

Differential Revision: D18746853

fbshipit-source-id: 9d01b24030f95ab5b4963660dc4ef471b5e0ad02