[CUDA][HIP] Enable kernel function return type deduction.

Summary:
- Even though only `void` is still accepted as the deduced return type,
  enabling deduction/instantiation on the return type allows more
  consistent coding.

Reviewers: tra, jlebar

Subscribers: cfe-commits, yaxunl

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68031

llvm-svn: 372898