[clang][Tooling] Fix assertion failure when processing CUDA files (#173762)
Running clang-tidy on CUDA files without specifying `--cuda-host-only`
or `--cuda-device-only` would trigger an assertion failure in
`Actions.size() > 1`, a related discussion:
https://github.com/llvm/llvm-project/pull/173699#discussion_r2649279975.
This occurred because the Clang Driver generates a single top-level
`OffloadAction` in `-fsyntax-only`, `-E`, `-M`. This commit removes the
overly strict assertions.
Closes #173777