Treat [1 x i8] zeroinitializer as a C string, placing such stuff into

mergeable string section. I don't see any bad impact of such decision
(rather then placing it into mergeable const section, as it was before),
but at least Darwin linker won't complain anymore.

The problem in LLVM is that we don't have special type for string constants
(like gcc does). Even more, we have two separate types: ConstatArray for non-null
strings and ConstantAggregateZero for null stuff.... It's a bit weird :)

llvm-svn: 63142