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.

llvm-svn: 208942