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

[Romaric Jodin via cfe-commits]

rjodinchr wrote:

clspv's `check-spirv` is what you want to test yes. But I was running within Google infra to try directly using the OpenCL-CTS.

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