[libclc] [libclc] Set OpenCL C version for each target (PR #135733)

[Wenju He via cfe-commits]

wenju-he wrote:

> Another question would be whether or not we should be advertising a 2.X or 3.X library if we don't have all the builtins? Is this a degradation? That might depend on downstream toolchains and if/how they react to versioning info in the LLVM IR.

We can incrementally add missing builtins. If we don't set the version, the newer builtins in libclc are not built at all and effectively dead code.

https://github.com/llvm/llvm-project/pull/135733