If the global variable is removed by the linker, then don't constant merge it

If the global variable is removed by the linker, then don't constant merge it
with other symbols.

An object in the __cfstring section is suppoed to be filled with CFString
objects, which have a pointer to ___CFConstantStringClassReference followed by a
pointer to a __cstring. If we allow the object in the __cstring section to be
merged with another global, then it could end up in any section. Because the
linker is going to remove these symbols in the final executable, we shouldn't
bother to merge them.
<rdar://problem/10564621>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147899 91177308-0d34-0410-b5e6-96231b3b80d8