PR19623: Implement typedefs of void.

This the LLVM portion that will allow Clang and other frontends to emit
typedefs of void by providing a null type for the typedef's underlying
type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207777 91177308-0d34-0410-b5e6-96231b3b80d8