Enable libcxx map test on FreeBSD again

The test used to trigger an assertion failure "Cannot get layout of
forward declarations!", but it no longer fails when built with
Clang 3.4.1 (system compiler) or 3.5 from SVN on FreeBSD.

llvm.org/pr17231


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218196 91177308-0d34-0410-b5e6-96231b3b80d8