LLVM 24: configure float-abi via module flag

[TimNN]

TargetOptions::FloatABIType was removed in llvm/llvm-project#215796

See llvm/llvm-project#212985 for the corresponding clang change.

The ModuleFlagMergeBehavior::Error was copied from the clang implementation.

This currently relies on the string representation of Rust's FloatAbi "target_spec_enum" matching LLVM. This seemed reasonable to me since the representation is quite straightforward ("soft" and "hard"), but let me know if you'd like an explicit to_llvm_float_abi_str, either defined in Rust or forwarding to llvm::FloatABI::getABITypeName.

Contrary to the clang implementation which only sets the module flag if it differs from the target default, we set it whenever the Rust target has an explicit float ABI set. See also discussion in #t-compiler/llvm > float-abi module flag behavior.

The added test won't actually run on Rust's own CI until that updates to LLVM 24.

@rustbot label llvm-main

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[TimNN]

@rustbot author

I noticed that I was a bit inconsistent with the LLVM version conditions - I think there's no real issue with applying the flag on LLVM versions that don't explicitly check it, but want to verify on Zulip.

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[TimNN]

@rustbot ready

Based on the Zulip thread, I've updated the PR to only apply the float-abi module flag on LLVM 24+.