Do not emit a UsedDirective for things in the llvm.used

Do not emit a UsedDirective for things in the llvm.used
list that have internal linkage; the linker doesn't need
or want this.  (These objects must still be preserved
at compile time, so just removing them from the llvm.used
list doesn't work.)  Should affect only Darwin.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@55722 91177308-0d34-0410-b5e6-96231b3b80d8