Fix a folly build failure under clang: MPMCQueueTest.cpp.

Fix a folly build failure under clang: MPMCQueueTest.cpp.

Summary:
[Folly] Fix a folly build failure under clang: MPMCQueueTest.cpp.

In clang-v3.4, there is a bug with the combination of a lambda expression inside a function template taking a template name (rather than a type name) as a template argument.

This diff, in the interest of building folly under clang-v3.4, extracts the lambda expression into a separate function so that the function template taking a template name as a template argument no longer has a lambda expression in it.

Test Plan: Build folly/test/MPMCQueueTest.cpp under clang.

Reviewed By: njormrod@fb.com

Subscribers: mathieubaudet, dougw

FB internal diff: D1446279

Tasks: 4723132