[SYCL][ROCm] Fix building libclc for AMD

[npmiller]

This file uses cl_mem_fence_flags, CLK_GLOBAL_MEM_FENCE,
CLK_LOCAL_MEM_FENCE, which are defined by clc.h.

It wasn't included because when this file was written the OpenCL headers
were implicitely included, this has changed so now including the header
is necessary. See the discussion on #4133 for more details.

[bader]

This file uses cl_mem_fence_flags, CLK_GLOBAL_MEM_FENCE,
CLK_LOCAL_MEM_FENCE, which are defined by clc.h.

These must be defined by the OpenCL compiler (see an example in clang/test/SemaOpenCL/fdeclare-opencl-builtins.cl), which builds barrier.cl.
I'm not sure I understand the purpose of clc.h to say which way to define these symbols is the right one, but by default I would say we should just rely on OpenCL compiler built-in capabilities.

Maybe the right way to include the header is to add -finclude-default-header instead of this include?
@Naghasan, do you have any thoughts on that?

[Naghasan]

These must be defined by the OpenCL compiler (see an example in clang/test/SemaOpenCL/fdeclare-opencl-builtins.cl), which builds barrier.cl.
I'm not sure I understand the purpose of clc.h to say which way to define these symbols is the right one, but by default I would say we should just rely on OpenCL compiler built-in capabilities.

libclc tries to be compatible with older clang versions, so might not have access to those definitions or may held unparsable attributes. Instead they provide their own definition to unify things. Also as it is defined by the OpenCL standard, so it is not really a moving target. It is annoying from our perspective, but that's how it is.

libclc recently prevent this inclusion by adding -fno-include-default-header and got included in the sycl branch with this pull #4133 (comment).

I think this patch is inline with the general design of libclc.

[bader]

@Naghasan, thank you for clarification.