Fix logic for not emitting no-dead-strip for some

Fix logic for not emitting no-dead-strip for some
objects in llvm.used (thanks Anton).  Makes visible
the magic 'l' prefix for symbols on Darwin which are
to be passed through the assembler, then removed at
linktime (previously all references to this had been
hidden in the ObjC FE code, oh well).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@55973 91177308-0d34-0410-b5e6-96231b3b80d8