[libclc] [libclc] Move sign to the CLC builtins library (PR #115699)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

> So is the sign function currently implemented in the OpenCL headers?

The sign function for SPIR-V/Mesa is currently implemented by libclc [here](https://github.com/llvm/llvm-project/blob/main/libclc/generic/lib/common/sign.cl). With this change, `sign` now calls `__clc_sign` which has the same implementation. We just need to do a bit of extra inlining of CLC functions into OpenCL functions to maintain more or less the same SPIR-V.

https://github.com/llvm/llvm-project/pull/115699