[PATCH] Make filesystem::path work with basic_string_view (P0392R0)

[Jonathan Wakely <jwakely at redhat dot com>]

This integrates string_view with experimental::filesystem::path, which
is not actually in the TS, but is required for std::filesystem. I've
made it work with experimental::string_view in C++14 mode, and with
std::string_view in C++17 mode. I suppose I could have made it work
with either in C++17 mode, but that would be a bit more complicated.

	* include/experimental/bits/fs_path.h (__is_path_src)
	(_S_range_begin, _S_range_end): Overload to treat string_view as a
	Source object.
	(path::operator+=, path::compare): Overload for basic_string_view.
	* testsuite/experimental/filesystem/path/construct/string_view.cc:
	New test.
	* testsuite/experimental/filesystem/path/construct/
	string_view_cxx17.cc: New test.

Tested x86_64-linux, committed to trunk.

Attachment: patch.txt
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