[libclc] [libclc] Create an internal 'clc' builtins library (PR #109985)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

CC @rjodinchr. I realise now that this idea may prove problematic for `clspv`/`clspv64` targets. If this idea were to taken further, things like OpenCL `minmag` would call `__clc_minmag` which would call `__clc_fabs` (e.g.), but I notice `fabs` is not implemented in `clspv*` libclc and it's left as external declarations. What exactly is happening with clspv libclc builtins? Same question for conversions, etc.

https://github.com/llvm/llvm-project/pull/109985