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

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

CC @rjodinchr - are you able to test this PR out for your needs, by any chance? It's quite intrusive so I'd like to be extra sure I haven't broken anything.

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