libstdc++: Make filesystem::path(path&&) always noexcept
Since r12-4065 std::basic_string is always nothrow-move-constructible,
so filesystem::path is too.
That also means that path::_S_convert(T) is noexcept when returning its
argument, because T is either a basci_string or basic_string_view, and
will be moved into the return value.
libstdc++-v3/ChangeLog:
* include/bits/fs_path.h (path(path&&)): Make unconditionally
noexcept.
(path::_S_convert(T)): Add condtional noexcept.