Don't forward declare environ on Windows

That seems to interact poorly with the environ and _environ macros
defined in MSVC's <stdlib.h>.

Also remove the incorrect comment about _NSGetEnviron().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180200 91177308-0d34-0410-b5e6-96231b3b80d8