Just because a GlobalVariable's initializer is [N x { i32, void ()* }] doesn't

Just because a GlobalVariable's initializer is [N x { i32, void ()* }] doesn't
mean that it has to be ConstantArray of ConstantStruct. We might have
ConstantAggregateZero, at either level, so don't crash on that.

Also, semi-deprecate the sentinal value. The linker isn't aware of sentinals so
we end up with the two lists appended, each with their "sentinals" on them.
Different parts of LLVM treated sentinals differently, so make them all just
ignore the single entry and continue on with the rest of the list.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@129307 91177308-0d34-0410-b5e6-96231b3b80d8