[amdgpu] Add codegen support for HIP dynamic shared memory.

Summary:
- HIP uses an unsized extern array `extern __shared__ T s[]` to declare
  the dynamic shared memory, which size is not known at the
  compile time.

Reviewers: arsenm, yaxunl, kpyzhov, b-sumner

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, dstuttard, tpr, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82496