Hoist the AllocaInfo struct to the top of the file.

This struct is nicely independent of everything else, and we already
needed a foward declaration here. It's simpler to just define it
immediately.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186780 91177308-0d34-0410-b5e6-96231b3b80d8