[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:

@chrisnc what are the next steps here?

Maybe one solution, for maximum compatibility, would be to not emit any warnings when `Options.FloatABIType` is left at `FloatABI::Default`, but if it is explicitly set to `FloatABI::Hard` and the required target features are missing, that should warn. (I assume `FloatABI::Soft` requires no target features.) `FloatABI::Hard` also seems to be incompatible with the `soft-float` target feature so that should probably also be a warning.

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