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

[Peter Smith via cfe-commits]

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@@ -85,6 +85,8 @@ class VectorType;
   //  ARMTargetLowering - ARM Implementation of the TargetLowering interface
 
   class ARMTargetLowering : public TargetLowering {
+    friend class ARMBaseTargetMachine; // For getEffectiveCallingConv().
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smithp35 wrote:

Instead of using a friend, I'm wondering if getEffectiveCallingConv() can be made a free function, possibly liberated from ARMISelLowering if it is going to be used in other places.

Looks like it would need the `Subtarget` and `getTM().isAAPCS_ABI()` passed through. 

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