Update cstddef after clang r207606.

r207606 changed the __need_foo macros to behave like they do with gcc: If they
are set, _only_ the __need_foo stuff gets defined.  As a consequence, cstddef
no longer defined "offsetof".  It looks like the __need_foo defines aren't
needed anymore, so just remove them.

Fixes PR19723.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@208942 91177308-0d34-0410-b5e6-96231b3b80d8