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

[Romaric Jodin via cfe-commits]

rjodinchr wrote:

I think for `clspv`, we would need to invert the condition to skip user-facing functions. By that I mean that `clspv` will need all the `convert_*` functions, and not the `__clc_convert_*_rte`. Then we will need to update `clspv` to support natively the function with the `__clc_` prefix.

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