[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:

> I'm not yet convinced that llvm is the right place for this error message. There's quite a lot of test changes that I presume needed to make and at least the LTO use case looks like we don't want to require extra information. I'd also prefer this to be a front-end error/warning as this is a front-end/source-level mistake, with better diagnostic control options available.

There's a *lot* of complicated logic required to determine, for each architecture, which target features affect the ABI in which ways. Are you suggesting that logic should be duplicated across all frontends? That seems like a huge waste of effort to me. Most frontends are inevitably going to get it subtly wrong, so in the end we have a ton more bugs than we did if there was a central location where such know-how could be properly encoded, and where there are enough people that can confidently answer ABI questions like this.

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