Reland: Fix and test inter-procedural register allocation for ARM
Add an explicit construction of the ArrayRef, gcc 5 and earlier don't
seem to select the ArrayRef constructor which takes a C array when the
construction is implicit.
Original commit message:
- Avoid a crash when IPRA calls ARMFrameLowering::determineCalleeSaves with a null RegScavenger. Simply not updating the register scavenger is fine because IPRA only cares about the SavedRegs vector, the acutal code of the function has already been generated at this point.
- Add a new hook to TargetRegisterInfo to get the set of registers which can be clobbered inside a call, even if the compiler can see both sides, by linker-generated code.
Differential revision: https://reviews.llvm.org/D64908