[libclc] [libclc] Move conversion builtins to the CLC library (PR #124727)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

I'm seeing several (82 out of 1045) conversions failures in the OpenCL-CTS. It's in saturating conversions from floating-point types to integers, and in regular float-to-float conversions. It isn't a result of this PR as they are already present on `main`.

I will merge this PR and investigate the failures next. They should be easier to fix on top of this PR. I wonder if the clspv-specific conversions are more correct.

https://github.com/llvm/llvm-project/pull/124727