[ManagedMemoryRewrite] Learn how to rewrite global arrays, allocas.

- If we have global arrays, we would like to rewrite them to global
  pointers which are allocated using `cudaMallocManaged`.

- If we have allocas in a function, we would like to rewrite them to
  heap-allocations with `cudaMallocManaged` and `cudaFree`.

- With these rewrite mechanisms, we can offload _any_ function to the
  GPU with no code rewrite whatsover.

Differential Revision: https://reviews.llvm.org/D36516

llvm-svn: 311080