Use a constexpr char* under MSVC for the constexpr StringPiece test
Use a constexpr char* under MSVC for the constexpr StringPiece test

Summary: MSVC can't handle the array as a constexpr value, but works fine with the char*.

Reviewed By: yfeldblum

Differential Revision: D3705189

fbshipit-source-id: e8208b3f2831a720095641f0e1e72ac63ed845a0

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