libstdc++: Fix dangling string_view in filesystem::path [PR102592]

libstdc++: Fix dangling string_view in filesystem::path [PR102592]

When creating a path from a pair of non-contiguous iterators we pass the
iterators to _S_convert(Iter, Iter). That function passes the iterators
to __string_from_range to get a contiguous sequence of characters, and
then calls _S_convert(const C*, const C*) to perform the encoding
conversions. If the value type, C, is char8_t, then no conversion is
needed and the _S_convert<char8_t>(const char8_t*, const char8_t*)
specialization casts the pointer to const char* and returns a
std::string_view that refs to the char8_t sequence. However, that
sequence is owned by the std::u8string rvalue returned by
__string_from_range, which goes out of scope when _S_convert(Iter, Iter)
returns. That means the std::string_view is dangling and we get
undefined behaviour when parsing it as a path.

The same problem does not exist for the path members taking a "Source"
argument, because those functions all convert a non-contiguous range
into a basic_string<C> immediately, using __effective_range(__source).
That means that the rvalue string returned by that function is still in
scope for the full expression, so the string_view does not dangle.

The solution for the buggy functions is to do the same thing, and call
__string_from_range immediately, so that the returned rvalue is still in
scope for the lifetime of the string_view returned by _S_convert. To
avoid reintroducing the same problem, remove the _S_convert(Iter, Iter)
overload that calls __string_from_range and returns a dangling view.

libstdc++-v3/ChangeLog:

	PR libstdc++/102592
	* include/bits/fs_path.h (path::path(Iter, Iter, format))
	(path::append(Iter, Iter), path::concat(Iter, Iter)): Call
	__string_from_range directly, instead of two-argument overload
	of _S_convert.
	(path::_S_convert(Iter, Iter)): Remove.
	* testsuite/27_io/filesystem/path/construct/102592.C: New test.