historic. libclc was ignored by most of the clang opencl work and nobody volunteered to switch it to clang provided headers.
Moreover, it has been stuck in clc 1.1/1.2 so most of the new features don't apply.
The main user is mesa/clover so that one needs to be switched to using clang implicit headers first
LLAsm path still can't use (any) headers so that part of this patch would be needed anyway.
OK, I see. Thanks for the clarification! Btw the functionality of version 1.1. and 1.2 is fairly complete in clang headers. So it should be relatively low risk if you switch to it. If there are any specific adjustments needed for libclc we can surely find a way to accommodate those.