[llvm] [openmp] [offload] Standalone build fixes (PR #118173)

[Michał Górny via llvm-commits]

mgorny wrote:

> Do you know if #117933 will conflict with anything here? It includes a header file cross-project.

We'll probably have to define `LLVM_MAIN_SRC_DIR`, but that shouldn't be a big problem.

Switched `PluginCommon` back to object library, and added updating `explicit_llvm_config` instead. Moved the `FrontendOffloading` dep to `PluginCommon`, as requested.

https://github.com/llvm/llvm-project/pull/118173