Disable the constexpr tests for findFirstSet, findLastSet and nextPowTwo under MSVC
Disable the constexpr tests for findFirstSet, findLastSet and nextPowTwo under MSVC

Summary: Because they aren't currently constexpr under MSVC.

Reviewed By: yfeldblum

Differential Revision: D3705095

fbshipit-source-id: ea266ebd9677fb2ba232476160f7c10a23954db9

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