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

folly/test/RangeTest.cpp

@@ -293,7 +293,7 @@ TEST(StringPiece, InvalidRange) {
   EXPECT_THROW(a.subpiece(6), std::out_of_range);
 }
 
-constexpr char helloArray[] = "hello";
+constexpr const char* helloArray = "hello";
 
 TEST(StringPiece, Constexpr) {
   constexpr StringPiece hello1("hello");