[Clang] Determine offloading architectures at Toolchain creation (#145799)
Summary:
Previously we had this weird disconnect where we would get some
offloading architectures beforehand and some later. This patch changes
it to where we just generate this information at Toolchain creation.
There's a few edge cases that will need to be cleaned up. Namely, we
don't handle the strange SPIR-V handling that mixes two separate
toolchains and we needed a pre-check to reject errors when inferring the
toolchain from `--offload-arch` in OpenMP.
Possible we could also use this information for some host defines if
needed.