Summary: [Folly] Use static const keys in AtomicHashArray::Config.

From https://github.com/facebook/folly/pull/264/, MSVC may have a hard time with certain expressions. D2284130 factored them out as `static constexpr` members, but Clang had a hard time with that in HPHP. This adds a test case that triggers the same failure to the Folly build. Not quite sure how this impacts MSVC though.

Reviewed By: @Gownta

Differential Revision: D2304346