Fix #1034: Fix std::string_view build with libstdc++ and libc++

[whyer6]

#1034

Clang 使用 libc++ 会使用 #include <string_view>
Clang 使用 libstdc++ 会使用 #include <experimental/string_view>

所以 PhotonLibOS/common/string_view.h 在 Clang + libstdc++ 情况下编译会造成找不到头文件<string_view>错误

[CLAassistant]

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[lihuiba]

Thanks for your contribution. Let me check the compatibility with our internal projects before approving.