[clang] clang: Add BoundArch argument to addClangTargetOptions (PR #196504)

[Joseph Huber via cfe-commits]

https://github.com/jhuber6 commented:

> The current process is kind of convoluted. TranslateArgs
synthesizes a -mcpu argument from BoundArch, and later
addClangTargetOptions re-parses that -mcpu argument each
time it wants the architecture. Add this argument so this
can be cleaned up in a future change.

Is this a limitation? It feels like that's the expected behavior once we're in a singular toolchain. The bound arch is kind of intended to be an intermediate step to generate this and we just do `getLastArg` on it.

https://github.com/llvm/llvm-project/pull/196504