[clang] [llvm] [ARM] Emit an error when the hard-float ABI is enabled but can't be used. (PR #111334)

[Ralf Jung via cfe-commits]

RalfJung wrote:

Yeah ideally LLVM would provide APIs where frontends can explicitly query that kind of information, and then provide nice errors with context. What this PR does is only the second-best think, in my eyes.

But very few frontends can afford having an expert for each and every architecture LLVM supports, so "do it in the frontend" is often simply not an option.

https://github.com/llvm/llvm-project/pull/111334