`<filesystem>`: Do we need to handle Volume GUID Paths?

[StephanTLavavej]

See Naming a Volume in the Win32 docs.

This previously came up in #4291 addressed by #4311, but that fix didn't alter how we parse paths.

Xiang Fan (@xiangfan-ms) observed that we never attempt to handle such paths:

STL/stl/inc/filesystem

Lines 569 to 572 in 2c9e3e7

	_NODISCARD inline bool _Is_drive_prefix_with_slash_slash_question(const wstring_view _Text) {
	// test if _Text starts with a \\?\X: prefix
	return _Text.size() >= 6 && _Text._Starts_with(LR"(\\?\)"sv) && _Is_drive_prefix(_Text.data() + 4);
	}

It's unclear to me how extensively we'd need to alter our code to handle volume GUID paths, but they appear to be cromulent so this appears to be a bug.