[AMDGPU] Emit metadata for hidden arguments for kernel enqueue

[AMDGPU] Emit metadata for hidden arguments for kernel enqueue

Identifies kernels which performs device side kernel enqueues and emit
metadata for the associated hidden kernel arguments. Such kernels are
marked with calls-enqueue-kernel function attribute by
AMDGPUOpenCLEnqueueKernelLowering pass and later on
hidden kernel arguments metadata HiddenDefaultQueue and
HiddenCompletionAction are emitted for them.

Differential Revision: https://reviews.llvm.org/D39255

llvm-svn: 316907