[PATCH] Fix ARM AAPCS regression caused by r211898

[Oliver Stannard]

We could simply disable the use of byval for ARM, as it is not required for ABI correctness. Two of my 5 most recent ABI-related commits have been directly related to byval arguments. Do you know what the purpose of byval is, and if there would be any downside to not using it? Does it allow for some optimisation at the IR level? Even if it does, it is probably not a very significant gain as it is only used for very large types passed by value, and the implementation in the ARM backend generates unnecessary stores to the stack in a lot of cases.

http://reviews.llvm.org/D4541