[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:

> there doesn't seem to be a mechanism to issue a warning from within LLVM codegen, only to error. 

RISC-V does `errs() << "Hard-float 'f' ABI can't be used for a target that "  ...` and that acts as a warning. It just prints a message to stderr. It's not pretty but it's better than generating code that has the wrong ABI and spending an hour debugging that...

Rust will always set `Options.FloatABIType` to a non-`Default` value for ARM-32.

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