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

[Michał Górny via cfe-commits]

mgorny wrote:

I'm guessing that both `amdgcn` and `nvptx` targets now require `clspv-generate_convert.cl`, but the targets for them are defined only when `clspv` targets are enabled. No clue whether it means that the targets need to be defined unconditionally, or `clc-convert.cl` skipped when `clspv` targets are disabled.

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