Requiring C++17 for macOS hosts

In PR-39216 we are replacing swift-stdlib-tool's usage of Foundation's FileManager and other APIs with some C++17 STL APIs. This means that building the Swift project will now require a C++17 enabled compiler, but only on macOS since this tool is only built on macOS hosts. Xcode 13 ships with a C++17 enabled version of Clang and is already listed as a requirement to build Swift on macOS.

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