Then you cannot use Rust and must settle for lack of safety. (A profoundly silly question -- if modern C++ is not an option for whatever reason, then Rust is doubly so.)
Just because the only existing Rust compiler uses LLVM, it doesn't mean another implementations will not surface.
So hypothetical the OS not targeted by clang, can still have a Rust compiler on the OS SDK, offering all safety guarantees from Rust.
For example, there are OSes and hardware architectures not supported by clang that have Ada and even Java AOT compilers.
If you want an option that works today without nonsense as you put it, then I would pick Ada over C++ for secure critical projects in such scenarios.
Companies like LDRA do exist, because just using clang isn't enough.
http://www.ldra.com/en/software-quality-test-tools/group/by-...
Also a reason why HIC exists http://www.codingstandard.com/