[libcxx] [test] Don't ask whether Incomplete& can be assigned to.

This is the subject of an active NB comment. Regardless of what the Working
Paper currently says, asking this question is morally wrong, because the
answer can change when the type is completed. C1XX now detects such
precondition violations and complains about them; perhaps Clang should too.

Fixes D28591.

llvm-svn: 292281