[PATCH] Fix AAPCS-VFP non-compliance when returning HFA from variadic function

[Oliver Stannard]

Hi,

According to the "Procedure Call Standard for the ARM Architecture", a
function which returns a struct larger than 4 bytes must pass the return
value in memory provided by the caller. This is true when using the base
AAPCS calling convention, and the AAPCS-VFP extension when the callee is
variadic. However, clang currently does not do the necessary transformation
to return the result in memory when using the AAPCS-VFP calling convention,
and when the struct is a homogeneous aggregate of floating-point types.

The attached patch corrects this behaviour. Note, that this change will
break ABI backwards-compatibility with previous versions of clang/llvm.

Oliver
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