Test that StdUniquePtrPrinter correctly prints std::unique_ptr objects
using the old layout, prior to the PR libstdc++/77990 changes.
The printer test for a valueless std::variant started to fail because
the PR libstdc++/87431 fix meant it no longer became valueless. Change
the test to use a type that is not trivially copyable, so that the
exception causes it to become valueless.
* testsuite/libstdc++-prettyprinters/compat.cc: Test printer support
for old std::unique_ptr layout.
* testsuite/libstdc++-prettyprinters/cxx17.cc: Fix std::variant test
to become valueless. Add filesystem::path tests.
From-SVN: r267743
