[libclc] [libclc][NFC] Move convert builtins from Python generator to .cl sources (PR #172634)

[Fraser Cormack via cfe-commits]

https://github.com/frasercrmck approved this pull request.

LGTM, the python code was always a pain to work with.

Shouldn't this PR remove `gen_convert.py`?

https://github.com/llvm/llvm-project/pull/172634