Revert "[lib/Headers] Define NULL as __DARWIN_NULL when on __APPLE__."

Per feedback by Doug, we should avoid platform-specific implementations
in lib/Headers as much as possible.

This reverts commit r178110.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178181 91177308-0d34-0410-b5e6-96231b3b80d8