(easy) Refactor FOLLY_LAMBDA_CONSTEXPR
(easy) Refactor FOLLY_LAMBDA_CONSTEXPR

Summary: This change was supposed to be in D32607012 (https://github.com/facebook/folly/commit/fc80a3aa0ad74179e3ce1808a18ff104d60d86ca), but didn't make it in.

Reviewed By: praihan

Differential Revision: D32928488

fbshipit-source-id: 1ae34297f42fe2cefc7e58a41787b7b3c5f158ae

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