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
Don't produce an empty llvm.compiler.used in LTO.

LTO was always creating an empty llvm.compiler.used. With this patch we
now first check if there is anything to be added first.

Unfortunately, there is no good way to test libLTO in isolation as it needs gold
or ld64, but there are bots doing LTO builds that found this problem.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180202 91177308-0d34-0410-b5e6-96231b3b80d8