libstdc++: Fix pretty printing of std::unique_ptr [PR103086]

Since std::tuple started using [[no_unique_address]] the tuple<T*, D>
member of std::unique_ptr<T, D> has two _M_head_impl subobjects, in
different base classes. That means this printer code is ambiguous:

    tuple_head_type = tuple_impl_type.fields()[1].type   # _Head_base
    head_field = tuple_head_type.fields()[0]
    if head_field.name == '_M_head_impl':
        self.pointer = tuple_member['_M_head_impl']

In older versions of GDB it happened to work by chance, because GDB
returned the last _M_head_impl member and std::tuple's base classes are
stored in reverse order, so the last one was the T* element of the
tuple. Since GDB 11 it returns the first _M_head_impl, which is the
deleter element.

The fix is for the printer to stop using an ambiguous field name and
cast the tuple to the correct base class before accessing the
_M_head_impl member.

Instead of fixing this in both UniquePointerPrinter and StdPathPrinter a
new unique_ptr_get function is defined to do it correctly. That is
defined in terms of new tuple_get and _tuple_impl_get functions.

It would be possible to reuse _tuple_impl_get to access each element in
StdTuplePrinter._iterator.__next__, but that already does the correct
casting, and wouldn't be much simpler anyway.

libstdc++-v3/ChangeLog:

	PR libstdc++/103086
	* python/libstdcxx/v6/printers.py (_tuple_impl_get): New helper
	for accessing the tuple element stored in a _Tuple_impl node.
	(tuple_get): New function for accessing a tuple element.
	(unique_ptr_get): New function for accessing a unique_ptr.
	(UniquePointerPrinter, StdPathPrinter): Use unique_ptr_get.
	* python/libstdcxx/v6/xmethods.py (UniquePtrGetWorker): Cast
	tuple to its base class before accessing _M_head_impl.