libstdc++: Refactor filesystem::path string conversions

This simplifies the logic of converting Source arguments and pairs of
InputIterator arguments into the native string format. For any input
that is a contiguous range of path::value_type (or char8_t for POSIX)
a string view can be created and the conversion can be done directly,
with no intermediate allocation. Previously some cases created a
basic_string unnecessarily, for example construction from a pair of
path::string_type::iterators, or a pair of non-const value_type*
pointers.

	* include/bits/fs_path.h (__detail::_S_range_begin)
	(__detail::_S_range_end, path::_S_string_from_iter): Replace with
	overloaded function template __detail::__effective_range.
	(__detail::__effective_range): New overloaded function template to
	create a basic_string or basic_string_view for an effective range.
	(__detail::__value_type_is_char): Use __detail::__effective_range.
	Do not use remove_const on value type.
	(__detail::__value_type_is_char_or_char8_t): Likewise.
	(path::path(const Source&, format))
	(path::path(const Source&, const locale&))
	(path::operator/=(const Source&), path::append(const Source&))
	(path::concat(const Source&)): Use __detail::__effective_range.
	(path::_S_to_string(InputIterator, InputIterator)): New function
	template to create a string view if possible, or string otherwise.
	(path::_S_convert): Add overloads that convert a string returned
	by __detail::__effective_range. Use if-constexpr to inline conversion
	logic from all overloads of _Cvt::_S_convert.
	(path::_S_convert_loc): Add overload that converts a string. Use
	_S_to_string to avoid allocation when possible.
	(path::_Cvt): Remove.
	(path::operator+=(CharT)): Remove indirection through path::concat.
	* include/experimental/bits/fs_path.h (path::_S_convert_loc): Add
	overload for non-const pointers, to avoid constructing a std::string.
	* src/c++17/fs_path.cc (path::_S_convert_loc): Replace conditional
	compilation with call to _S_convert.

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