[AMDGPU] Fix lowering enqueue kernel when kernel has no name

Since the enqueued kernels have internal linkage, their names may be dropped.
In this case, give them unique names __amdgpu_enqueued_kernel or
__amdgpu_enqueued_kernel.n where n is a sequential number starting from 1.

Differential Revision: https://reviews.llvm.org/D44322

llvm-svn: 327291