libstdc++: Fix path::generic_string allocator handling (PR 94242)

libstdc++: Fix path::generic_string allocator handling (PR 94242)

It's not possible to construct a path::string_type from an allocator of
a different type. Create the correct specialization of basic_string, and
adjust path::_S_str_convert to use a basic_string_view so that it is
independent of the allocator type.

	PR libstdc++/94242
	* include/bits/fs_path.h (path::_S_str_convert): Replace first
	parameter with basic_string_view so that strings with different
	allocators can be accepted.
	(path::generic_string<C,T,A>()): Use basic_string object that uses the
	right allocator type.
	* testsuite/27_io/filesystem/path/generic/94242.cc: New test.
	* testsuite/27_io/filesystem/path/generic/generic_string.cc: Improve
	test coverage.