[libcxx] [test] D26813: allocator<const T> is non-Standard.

N4582 17.6.3.5 [allocator.requirements] says that allocators are given
cv-unqualified object types, and N4582 20.9.9 [default.allocator]
implies that allocator<const T> is ill-formed (due to colliding
address() overloads). Therefore, tests for allocator<const T>
should be marked as libcxx-specific (if not removed outright).

llvm-svn: 287381