Type safe version of MachinePassRegistry

Type safe version of MachinePassRegistry

Previous version used type erasure through a `void* (*)()` pointer,
which triggered gcc warning and implied a lot of reinterpret_cast.

This version should make it harder to hit ourselves in the foot.

Differential revision: https://reviews.llvm.org/D54203

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346522 91177308-0d34-0410-b5e6-96231b3b80d8