[clang] [clang] Add isOffloadingTarget function to LangOpts (PR #126956)

[Nick Sarnie via cfe-commits]

sarnex wrote:

> The target isn't part of the language, why is it in LangOpts?

If you have a better suggestion I'm all areas, we seem to already have similar stuff in `LangOpts` such as `OMPTargetTriples`, `OMPHostIRFile`, `GPUDefaultStream`, and `CUID`.

https://github.com/llvm/llvm-project/pull/126956